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;

namespace Atcoder0629
{
    class Program
    {
        static void Main(string[] args)
        {
            string S = Console.ReadLine();
            char c = S[0];
            for(int i = 1;i < 4; i++)
            {
                if (S[0] == S[i])
                {
                    S = S.Remove(i,1);
                    S = S.Remove(0, 1);
                    break;
                }
            }
            if (S[0] == S[1] && S[0]!=c)
            {
                Console.WriteLine("Yes");
            }
            else
            {
                Console.WriteLine("No");
            }
            Console.ReadLine();
            return;
        }
    }
}