input: def find_substring(source: str, substring: str) -> int:
      """This function returns the start index of a substring in given string """
      if substring not in source:
          return -1
      i = 0
      while i < len(source):
          if source[i] == substring[0]:
              flag = True
              for j in range(len(substring)):
                  if substring[j] != source[i + j]:
                      flag = False
                      break
              if flag:
                  return i
          i += 1
      return -1
  
  # Example cases:
  # find_substring('Hello World', 'World') returns 6
  # find_substring('Python is fun', 'is') returns 7
  # find_substring('Empty', 'Full') returns -1
output:     if substring not in source:
          return -1
      i = 0
      while i < len(source):
          if source[i] == substring[0]:
              flag = True
              for j in range(len(substring)):
                  if i + j >= len(source) or substring[j] != source[i + j]:
                      flag = False
                      break
              if flag:
                  return i
          i += 1
      return -1