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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            const string ANSUCCESS = "No";
            const string SUCCESS = "Yes";
            var input = Console.ReadLine();
            var first = input.First();

            var input2 = input.Replace(first, ' ').Replace(" ", "");
            if (input2.Length != 2)
            {
                Console.WriteLine(ANSUCCESS);
                return;
            }
            var input3 = input.Replace(input2.First(), ' ').Replace(" ", "");
            if (input3.Length != 0)
            {
                Console.WriteLine(ANSUCCESS);
                return;
            }

            Console.WriteLine(SUCCESS);

        }
    }
}