diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
new file mode 100644
index 0000000..69fada0
--- /dev/null
+++ b/.github/workflows/ci.yaml
@@ -0,0 +1,65 @@
+name: CI
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, ready_for_review]
+  push:
+    branches: [main]
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  checks:
+    name: Lint / Format Check / Tests
+    runs-on: ubuntu-latest
+    if: ${{ github.event_name != 'push' || github.ref == 'refs/heads/main' }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        id: py
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install Poetry
+        uses: abatilo/actions-poetry@v3
+        with:
+          poetry-version: "2.2.0"
+
+      - name: Enable in-project venv
+        run: poetry config virtualenvs.in-project true
+
+      - name: Cache Poetry venv
+        uses: actions/cache@v4
+        with:
+          path: .venv
+          key: venv-${{ runner.os }}-${{ steps.py.outputs.python-version }}-${{ hashFiles('poetry.lock') }}
+          restore-keys: |
+            venv-${{ runner.os }}-
+
+      - name: Install dependencies
+        run: poetry install --no-interaction
+
+      # ---- Static checks (no file modifications in CI) ----
+      - name: Ruff lint
+        run: poetry run ruff check .
+
+      - name: Ruff format (check only)
+        run: poetry run ruff format --check .
+
+      # ---- Run tests with coverage threshold ----
+      - name: Run tests with coverage
+        run: |
+          poetry run pytest \
+            -q --maxfail=1 --disable-warnings \
+            --cov=src --cov-report=term-missing \
+            --cov-report=xml:coverage.xml \
+            --cov-fail-under=100
diff --git a/README.md b/README.md
index af2eb1b..1fd278f 100644
--- a/README.md
+++ b/README.md
@@ -4,9 +4,12 @@
 
 **Workflow**:
 1. Fork this repository to your own GitHub account.
-2. Create a new branch from `develop` and name it `feature/student1name_student2name`.
-3. Clone the repo to AWS VM.
-4. Implement the functions until all `pytest` tests pass.
-5. Push your code to your Github repo.
+2. Clone the repo to AWS VM.
+3. Run poetry install to install dependencies
+4. Run poetry run pre-commit install to install the pre-commit hook.
+5. Create a new branch from `develop` and name it `feature/student1name_student2name`.
+7. Implement the functions until all `pytest` tests pass.
+8. Make the pre-commit checks all pass and commit the code.
+9. Push your code to your Github repo.
 5. Submit a Pull Request back to the `develop` branch of the main repo. CI must pass.
 6. Wait for code review and grading.
diff --git a/poetry.lock b/poetry.lock
index 4177cc6..5b32389 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -38,7 +38,7 @@ version = "0.4.6"
 description = "Cross-platform colored terminal text."
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,!=3.5.*,!=3.6.*,>=2.7"
-groups = ["main"]
+groups = ["main", "dev"]
 markers = "sys_platform == \"win32\""
 files = [
     {file = "colorama-0.4.6-py2.py3-none-any.whl", hash = "sha256:4f1d9991f5acc0ca119f9d443620b77f9d6b33703e51011c16baf57afb285fc6"},
@@ -260,7 +260,7 @@ version = "2.1.0"
 description = "brain-dead simple config-ini parsing"
 optional = false
 python-versions = ">=3.8"
-groups = ["main"]
+groups = ["main", "dev"]
 files = [
     {file = "iniconfig-2.1.0-py3-none-any.whl", hash = "sha256:9deba5723312380e77435581c6bf4935c94cbfab9b1ed33ef8d238ea168eb760"},
     {file = "iniconfig-2.1.0.tar.gz", hash = "sha256:3abbd2e30b36733fee78f9c7f7308f2d0050e88f0087fd25c2645f63c773e1c7"},
@@ -393,7 +393,7 @@ version = "25.0"
 description = "Core utilities for Python packages"
 optional = false
 python-versions = ">=3.8"
-groups = ["main"]
+groups = ["main", "dev"]
 files = [
     {file = "packaging-25.0-py3-none-any.whl", hash = "sha256:29572ef2b1f17581046b3a2227d5c611fb25ec70ca1ba8554b24b0e69331a484"},
     {file = "packaging-25.0.tar.gz", hash = "sha256:d443872c98d677bf60f6a1f2f8c1cb748e8fe762d2bf9d3148b5599295b0fc4f"},
@@ -434,7 +434,7 @@ version = "1.6.0"
 description = "plugin and hook calling mechanisms for python"
 optional = false
 python-versions = ">=3.9"
-groups = ["main"]
+groups = ["main", "dev"]
 files = [
     {file = "pluggy-1.6.0-py3-none-any.whl", hash = "sha256:e920276dd6813095e9377c0bc5566d94c932c33b27a3e3945d8389c374dd4746"},
     {file = "pluggy-1.6.0.tar.gz", hash = "sha256:7dcc130b76258d33b90f61b658791dede3486c3e6bfb003ee5c9bfb396dd22f3"},
@@ -484,7 +484,7 @@ version = "2.19.2"
 description = "Pygments is a syntax highlighting package written in Python."
 optional = false
 python-versions = ">=3.8"
-groups = ["main"]
+groups = ["main", "dev"]
 files = [
     {file = "pygments-2.19.2-py3-none-any.whl", hash = "sha256:86540386c03d588bb81d44bc3928634ff26449851e99741617ecb9037ee5ec0b"},
     {file = "pygments-2.19.2.tar.gz", hash = "sha256:636cb2477cec7f8952536970bc533bc43743542f70392ae026374600add5b887"},
@@ -499,7 +499,7 @@ version = "8.4.2"
 description = "pytest: simple powerful testing with Python"
 optional = false
 python-versions = ">=3.9"
-groups = ["main"]
+groups = ["main", "dev"]
 files = [
     {file = "pytest-8.4.2-py3-none-any.whl", hash = "sha256:872f880de3fc3a5bdc88a11b39c9710c3497a547cfa9320bc3c5e62fbf272e79"},
     {file = "pytest-8.4.2.tar.gz", hash = "sha256:86c0d0b93306b961d58d62a4db4879f27fe25513d4b969df351abdddb3c30e01"},
@@ -940,5 +940,5 @@ test = ["covdefaults (>=2.3)", "coverage (>=7.2.7)", "coverage-enable-subprocess
 
 [metadata]
 lock-version = "2.1"
-python-versions = ">=3.12"
-content-hash = "c6fe4b288327b501a15d79c555e296323d3da6a16b612a92ac74271600991431"
+python-versions = "^3.12"
+content-hash = "ecd46f9c7df0b8d74ebc590edd798e935aba1f8c50228d60e3633baa8e680c89"
diff --git a/pyproject.toml b/pyproject.toml
index 4274d13..8408e51 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -16,6 +16,20 @@ dependencies = [
     "pre-commit (>=4.3.0,<5.0.0)"
 ]
 
+[tool.poetry]
+name = "arithmetic"
+version = "0.1.0"
+description = "Toy arithmetic package"
+authors = ["Your Name <you@example.com>"]
+readme = "README.md"
+packages = [{ include = "src/arithmetic" }]
+
+[tool.poetry.dependencies]
+python = "^3.12"
+
+
+[tool.poetry.group.dev.dependencies]
+pytest = "^8.4.2"
 
 [build-system]
 requires = ["poetry-core>=2.0.0,<3.0.0"]
diff --git a/src/arithmetic/arithmetic.py b/src/arithmetic/arithmetic.py
index 86f6960..72fd20a 100644
--- a/src/arithmetic/arithmetic.py
+++ b/src/arithmetic/arithmetic.py
@@ -13,7 +13,7 @@ def add_numbers(a: int, b: int) -> int:
     Returns:
         Sum of a and b.
     """
-    raise NotImplementedError
+    return a + b
 
 
 def factorial(n: int) -> int:
@@ -28,7 +28,12 @@ def factorial(n: int) -> int:
     Raises:
         ValueError: if n is negative
     """
-    raise NotImplementedError
+    if n < 0:
+        raise ValueError("Factorial is not defined for negative numbers")
+    result = 1
+    for i in range(2, n + 1):
+        result *= i
+    return result
 
 
 def is_prime(n: int) -> bool:
@@ -43,4 +48,10 @@ def is_prime(n: int) -> bool:
     Returns:
         True if n is prime; otherwise False.
     """
-    raise NotImplementedError
+    if n <= 1:
+        return False
+    else:
+        for i in range(2, n):
+            if n % i == 0:
+                return False
+        return True