input: Write a python function to find the starting position of the longest contiguous substring in alphabetical order.
  assert longest_alphabet("abcabcdefg") == 0
  assert longest_alphabet("abcaxyz") == 3
  assert longest_alphabet("zxyabcde") == 3
output: def longest_alphabet(s):
      max_len = 0
      max_start = 0
      current_start = 0
      current_len = 1
      
      for i in range(1, len(s)):
          if s[i] >= s[i - 1]:
              current_len += 1
          else:
              if current_len > max_len:
                  max_len = current_len
                  max_start = current_start
              current_start = i
              current_len = 1
      
      if current_len > max_len:
          max_len = current_len
          max_start = current_start
          
      return max_start