input: 
  def triangle_area(a, b, c):
      '''
      Given the lengths of the three sides of a triangle. Return the area of
      the triangle rounded to 2 decimal points if the three sides form a valid triangle. 
      Otherwise return -1
      Three sides make a valid triangle when the sum of any two sides is greater 
      than the third side.
      Example:
      triangle_area(3, 4, 5) == 6.00
      triangle_area(1, 2, 10) == -1
      '''
  
output:     if a + b <= c or a + c <= b or b + c <= a:
          return -1
      s = (a + b + c) / 2
      area = (s * (s - a) * (s - b) * (s - c)) ** 0.5
      return round(area, 2)