input: Write a function to validate if a string represents a decimal number with exactly two decimal places.
  assert is_valid_decimal('123.11')==True
  assert is_valid_decimal('e666.86')==False
  assert is_valid_decimal('3.124587')==False
output: def is_valid_decimal(s):
      if '.' not in s:
          return False
      parts = s.split('.')
      if len(parts) != 2:
          return False
      if not parts[0].isdigit() or not parts[1].isdigit():
          return False
      return len(parts[1]) == 2