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;
using System.Collections.Generic;

class Program {
    static void Main (string[] args) {
        string S = Console.ReadLine ();
        string result = "Yes";
        int j = 0;
        int[] cnt = new int[4] { 0, 0, 0, 0 };

        foreach (var s in S) {

            for (int i = 0; i < 4; i++) {
                if (s == S[i]) cnt[j]++;
            }
            if (cnt[j] != 2) result = "No";
            j++;
        }
        Console.WriteLine (result);

    }
}