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.Linq;
using System.Collections.Generic;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            char[] inp = input.ToCharArray();
            char[] charset = new char[4];
            charset[0] = inp[0];
            charset[1] = inp[0];
            int[] count = new int[2];
            count[0] = 1;
            for (int i = 1; i < 4; i++)
            {
                if (inp[i] == charset[0]) count[0]++;
                else if (inp[i] == charset[1]) count[1]++;
                else
                {
                    charset[1] = inp[i];
                    count[1] = 1;
                }
            }
            if (count[0] == 2 && count[1] == 2)
            {
                Console.WriteLine("Yes");
            }
            else
            {
                Console.WriteLine("No");
            }
        }
    }

}