output_description: Print the length of the longest _ACGT string_ that is a substring of S.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      S
attempt: def actual(s):
      sub_string_list = []
  
      N = len(s)
      for i in range(N):
          for j in range(N - 1):
              sub_string_list.append(s[i:j])
  
      set_ACGT = {"A", "C", "G", "T"}
      length_list = []
  
      for sub_string in sub_string_list:
          if set(sub_string).issubset(set_ACGT):
              length_list.append(len(sub_string))
  
      return max(length_list)
  
  
  s = input()
  print(actual(s))
  
problem_description: Statement
  
  You are given a string S consisting of uppercase English letters. Find the
  length of the longest _ACGT string_ that is a substring (see Notes) of S.
  
  Here, a ACGT string is a string that contains no characters other than `A`,
  `C`, `G` and `T`.
samples: [{"input": "ATCODER", "output": "3\n    \n\nAmong the ACGT strings that are substrings of `ATCODER`, the longest one is\n`ATC`.\n\n* * *"}, {"input": "HATAGAYA", "output": "5\n    \n\nAmong the ACGT strings that are substrings of `HATAGAYA`, the longest one is\n`ATAGA`.\n\n* * *"}, {"input": "SHINJUKU", "output": "0\n    \n\nAmong the ACGT strings that are substrings of `SHINJUKU`, the longest one is\n`` (the empty string)."}]