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;
using System.Text;
using System.Numerics;

namespace SolutionCS
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();

            int diff = -1;
            int count = 0;
            for (int i = 1; i < 4; i++)
            {
                if (s[0] == s[i])
                {
                    count++;
                }
                else
                {
                    if ( diff <0)
                    {
                        diff = i;
                    }
                }
            }

            if (count != 1 || diff < 0)
            {
                Console.WriteLine("No");
                return;
            }

            count = 0;
            for (int i = 0; i < 4; i++)
            {
                if ( i == diff)
                {
                    continue;
                }

                if ( s[diff] == s[i])
                {
                    count++;
                }
            }

            if (count != 1 )
            {
                Console.WriteLine("No");
                return;
            }

            Console.WriteLine("Yes");
        }
    }
}