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;
public class Hello{
    public static void Main(){
        string s = Console.ReadLine();
        for (int i = 0;i<4;i++){
            int cnt = 0;
            for (int j = 0;j<4;j++){
                if (s[j]==s[i]){
                    cnt++;
                }
            }
            if (cnt !=2){
               Console.WriteLine("No");
               Environment.Exit(0);
            }
        }
        Console.WriteLine("Yes");
    }
}