Problem:
Score : 100 points

Problem 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.

Constraints

The length of S is 4.

S consists of uppercase English letters.

Input

Input is given from Standard Input in the following format:

S

Output

If S consists of exactly two kinds of characters which both appear twice in S, print Yes; otherwise, print No.

Sample Input 1

ASSA

Sample Output 1

Yes

S consists of A and S which both appear twice in S.

Sample Input 2

STOP

Sample Output 2

No

Sample Input 3

FFEE

Sample Output 3

Yes

Sample Input 4

FREE

Sample Output 4

No

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Text.RegularExpressions;

namespace AtCoder {
  class Program {
    static void Main(string[] args) {
      string S = Console.ReadLine();
      Regex[] regexs = new Regex[] {
        new Regex(@"(.)\1(.)\2"),
        new Regex(@"(.)(.)\1\2"),
        new Regex(@"(.)(.)\2\1")
      };
      foreach(var regex in regexs) {
        Match match = regex.Match(S);
        if(match.Success && match.Groups[1].Value != match.Groups[2].Value){
          Console.WriteLine("Yes");
          return;
        }
      }
      Console.WriteLine("No");
    }
  }
}