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;

namespace cs
{
    public class Program
    {
        public static void Main(string[] args)
        {
            
            var input = Console.ReadLine().ToCharArray();

            char a;
            char b = '\0';
            int aC = 1;
            int bC = 0;

            a = input[0];

            for(int i = 1; i< 4; i++){
                if(a == input[i]){
                    aC++;
                } 
                else if (b == '\0'){
                    b = input[i];
                    bC++;
                }else if( b == input[i]){
                    bC++;
                }
            }

            Console.WriteLine((aC == 2 && aC == bC) ? "Yes" : "No");
            
            
            
/*
            Console.WriteLine();
            Console.WriteLine("続行するにはアレ");*/
            //Console.ReadKey();
           
        }

        public static long GCD(long n, long m){
            
            if(m % n == 0) return n;
            else return GCD(m%n, n);

        }

        public static long LCD(long n, long m){

            return m * n / GCD(n,m);
        }
    }
}