input: Write a function to find a substring within a given string and return the starting index of the substring. If the substring is not found, return -1.
  
  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
  
  assert find_substring("hello world", "world")==6
  assert find_substring("hello world", "hello")==0
  assert find_substring("hello world", "o")==4
  assert find_substring("hello world", "planet")==-1
  assert find_substring("hello world", "rld")==8
output: def find_substring(source, substring):
      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