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..28cbed8 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -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 = "2208d21945ff72a894dfa1b053afd56efc83d7e50a8d39b29c814fdc7eb94a3e"
diff --git a/pyproject.toml b/pyproject.toml
index 4274d13..df128a4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -16,6 +16,17 @@ 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"
+
 
 [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..c104959 100644
--- a/src/arithmetic/arithmetic.py
+++ b/src/arithmetic/arithmetic.py
@@ -1,46 +1,37 @@
-"""arithmetic functions."""
-
-from __future__ import annotations
+# src/arithmetic/arithmetic.py
 
+from typing import Optional
 
 def add_numbers(a: int, b: int) -> int:
-    """Return the sum of two integers.
-
-    Args:
-        a: first integer
-        b: second integer
-
-    Returns:
-        Sum of a and b.
-    """
-    raise NotImplementedError
+    """Return the sum of two integers."""
+    return a + b
 
 
 def factorial(n: int) -> int:
-    """Compute the factorial of n.
-
-    Args:
-        n: non-negative integer
-
-    Returns:
-        n! as int
+    """Compute n! for a non-negative integer n.
 
     Raises:
-        ValueError: if n is negative
+        ValueError: if n is negative.
     """
-    raise NotImplementedError
+    if n < 0:
+        raise ValueError("factorial is undefined for negative integers")
+    result = 1
+    for i in range(2, n + 1):
+        result *= i
+    return result
 
 
 def is_prime(n: int) -> bool:
-    """Check whether n is a prime number.
-
-    A prime number is an integer greater than 1 that has no positive
-    divisors other than 1 and itself.
-
-    Args:
-        n: integer to test
-
-    Returns:
-        True if n is prime; otherwise False.
-    """
-    raise NotImplementedError
+    """Return True if n is prime, else False."""
+    if n <= 1:
+        return False
+    if n <= 3:
+        return True
+    if n % 2 == 0 or n % 3 == 0:
+        return False
+    i = 5
+    while i * i <= n:
+        if n % i == 0 or n % (i + 2) == 0:
+            return False
+        i += 6
+    return True