input: def find_substring(source: str, substring: str) -> int:
      """This function returns the start index of a substring in the given string.
      >>> find_substring("HelloWorld", "World")
      5
      >>> find_substring("HelloWorld", "Planet")
      -1
      >>> find_substring("PythonProgramming", "thon")
      2
      """
output:     for i in range(len(source)):
          if source[i:i+len(substring)] == substring:
              return i
      return -1