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;

public class CSharp
{
    public static void Main()
    {
        Kyopuro chokudai = new Kyopuro();
        chokudai.AtCoder();
    }
}

public class Kyopuro
{

    int n, m, k;

    public void AtCoder()
    {
        string[] input;
        input = Console.ReadLine().Split(' ');
        string s = input[0];
        var c = new List<char>();
        int x = 1;
        bool f = false;
        bool g = false;
        c.Add(s[0]);
        for(int i=1; i<4; i++)
        {
            g = false;
            for(int j=0; j<c.Count; j++)
            {
                if(s[i] == c[j])
                {
                    g = true;                   
                }                
            }
            if (!g)
            {
                x++;
                c.Add(s[i]);
            }
        }
        if (x == 2)
        {
            x = 0;
            for(int i=1; i<4; i++)
            {
                if (s[0] == s[i]) x++;
            }
            if (x == 1) f = true;
        }

        if (f) Console.WriteLine("Yes");
        else Console.WriteLine("No");
    }
    public static void DebugWL(object x) { System.Diagnostics.Debug.WriteLine(x); }
}