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;

class FiftyFifty
{
    static void Main(string[] args)
    {
        string input = Console.ReadLine();
        string answer = "";
        var k = 1;

        for (var i = 0; i <= 1; i++)
        {
            var count = 0;
            for (int j = k; j <= 3; j++)
            {
                if (input[i] == input[j])
                {
                    count++;
                }
            }

            if (count == 1)
            {
                answer = "Yes";
            }
            //同じ字が3文字か1文字の場合
            if (count == 2 || count == 0)
            {
                answer = "No";
                break;
            }
            k++;
        }
        Console.WriteLine(answer);
    }
}