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;

class Program
{
    public static void Main(string[] args)
    {
        string input = Console.ReadLine();

        char temp1 = input[0];
        int temp1Num = 1;
        char temp2 = 'z';
        int temp2Num = 0;

        for (int i = 1; i < input.Length; i++)
        {
            if (input[i] == temp1)
            {
                temp1Num++;
            }
            else if (input[i] != temp1 && temp2 == 'z')
            {
                temp2 = input[i];
            }
        }

        for (int i = 0; i < input.Length; i++)
        {
            if (input[i] == temp2)
            {
                temp2Num++;
            }
        }

        if (temp1Num == 2 && temp2Num == 2)
        {
            Console.WriteLine("Yes");
        }
        else
        {
            Console.WriteLine("No");
        }
    }
}