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;
using System.Collections.Generic;

class DivideTheProblems
{
    static void Main(string[] args)
    {
        string input = Console.ReadLine();
        string answer = "";
        char[] array = new char[input.Length];

        for (int i = 0; i < input.Length; i++)
        {
            array[i] = input[i];
        }

        Array.Sort(array);

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