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
  
  # Test cases
  # find_substring('Hello World', 'World') == 6
  # find_substring('Hello World', 'llo') == 2
  # find_substring('Hello World', 'Python') == -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