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.Collections.Generic;
class Program
    {
        static void Main(string[] args)
    	{
            string s = Console.ReadLine();
            char[] c = s.ToCharArray();
            Array.Sort(c);
            if (c[0] == c[1] && c[1] != c[2] && c[2] == c[3])
            {
                Console.WriteLine("Yes");
            }
            else Console.WriteLine("No");
            
            
        }
    
    }
    //mcs Main.cs
    //mono Main.exe