output_description: If S consists of exactly two kinds of characters which both appear twice in S,
  print `Yes`; otherwise, print `No`.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      S
attempt: C = set(s_i for s_i in input())
  print("Yes" if len(C) == 2 else "No")
  
problem_description: Statement
  
  You are given a 4-character string S consisting of uppercase English letters.
  Determine if S consists of exactly two kinds of characters which both appear
  twice in S.
samples: [{"input": "ASSA", "output": "Yes\n    \n\nS consists of `A` and `S` which both appear twice in S.\n\n* * *"}, {"input": "STOP", "output": "No\n    \n\n* * *"}, {"input": "FFEE", "output": "Yes\n    \n\n* * *"}, {"input": "FREE", "output": "No"}]