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 a
{
    public static void Main()
    {
        string s = Console.ReadLine();
        if (s[0] == s[1])
        {
            if (s[2] == s[3])
            {
                Console.WriteLine("Yes");
            }
            else
            {
                Console.WriteLine("No");

            }
        }
        if (s[0] == s[2])
        {
            if (s[1] == s[3])
            {
                Console.WriteLine("Yes");
            }
            else
            {
                Console.WriteLine("No");
            }
        }
        if (s[0] == s[3])
        {
            if (s[1] == s[2])
            {
                Console.WriteLine("Yes");
            }
            else
            {
                Console.WriteLine("No");
            }
        }
        if (s[1] == s[2])
        {
            if (s[0] == s[3])
            {
                Console.WriteLine("Yes");
            }
            else
            {
                Console.WriteLine("No");
            }
        }
        if (s[1] == s[3])
        {
            if (s[0] == s[2])
            {
                Console.WriteLine("Yes");
            }
            else
            {
                Console.WriteLine("No");
            }
        }
        if (s[2] == s[3])
        {
            if (s[0] == s[1])
            {
                Console.WriteLine("Yes");
            }
            else
            {
                Console.WriteLine("No");
            }
        }

    }
}