input: def find_substring(source: str, substring: str) -> int:
      """This function returns the start index of a substring in a given string.
      If the substring is not found, it returns -1.
      >>> find_substring("hello world", "world")
      6
      >>> find_substring("apple", "orange")
      -1
      """
output:     for i in range(len(source)):
          if source[i:i+len(substring)] == substring:
              return i
      return -1