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 Program
{
    static void Main(string[] args)
    {
        string S = Console.ReadLine();
        char[] charS = S.ToCharArray();
        bool ans = false;
        for(int i = 1; i < 4; i++)
        {
            if(charS[0] == charS[i])
            {
                if(i == 3)
                {
                    if (charS[1] == charS[2])
                    {
                        ans = true;
                    }
                }
                else if(i == 2)
                {
                    if(charS[1] == charS[3])
                    {
                        ans = true;
                    }
                }
                else
                {
                    if(charS[2] == charS[3])
                    {
                        ans = true;
                    }
                }
            }
        }
        if (ans)
        {
            Console.WriteLine("Yes");
        }
        else
        {
            Console.WriteLine("No");
        }
    }
}