diff --git a/demo.py b/demo.py
index 4592eec..b78225e 100644
--- a/demo.py
+++ b/demo.py
@@ -1,5 +1,21 @@
+"""
+Demo module for circle area calculation.
+"""
+
+import math
 
 
 def circle_area(radius):
-    """Returns the area of the circle of given radius"""
-    pass  # YOUR CODE HERE
+    """
+    Calculate the area of a circle.
+
+    Args:
+        radius (int or float): Radius of the circle
+
+    Returns:
+        float: Area of the circle or 0 if radius is negative
+    """
+    if radius < 0:
+        return 0
+
+    return math.pi * radius ** 2