func_code: def index(str, letter1, letter2):
      i = 0
      while i < len(str):
          if str[i] == letter1:
              return i
          i += 1
      return '-1'
      i = 0
      while i < len(str):
          if str[i] == letter2:
              return i
          i += 1
      return '-1'