input: def find_substring(source, substring):
      '''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
  
      """
      >>> find_substring("This is a test", "test")
      10
      >>> find_substring("Python programming", "Python")
      0
      >>> find_substring("Hello World", "lo")
      3
      >>> find_substring("This is a test", "notfound")
      -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 substring[j] != source[i+j]:
                      flag = False
                      break
              if flag:
                  return i
          i += 1
      return -1