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;

namespace abc132A
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            string A = Console.ReadLine();
            string[] B = new string[4];
            int C = 0;
            for(int i = 0; i < 4; i++)
            {
                B[i] = A.Substring(i, 1);
                for(int r = 0; r < 4; r++)
                {
                    B[r] = A.Substring(r, 1);
                    if (B[i] == B[r])
                        C = C + 1;
                }
            }
            C = (C - 4) / 2;
            Console.WriteLine(C);
        }
    }
}