input: Write a function to compute the area of a circle given its radius. If the radius is negative, return `"Invalid radius"`.
  assert area_circle(5) == 78.54
  assert area_circle(0) == 0.00
  assert area_circle(-3) == "Invalid radius"
output: def area_circle(radius):
      if radius < 0:
          return "Invalid radius"
      return 3.14159 * radius * radius