input: Write a function to calculate the area of a circle given its radius. If the radius is less than or equal to zero, return a message indicating that the radius must be positive.
  assert area_circle(5)==78.54
  assert area_circle(0)=='Radius must be positive'
  assert area_circle(-3)=='Radius must be positive'
  assert area_circle(2.5)==19.63
output: def area_circle(radius):
      if radius <= 0:
          return 'Radius must be positive'
      return round(3.14159 * radius * radius, 2)