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;
public class ABC132A {
    public static void Main() {
        int n = 0, nA = 0, nB = 0;
        char cA = '\0', cB = '\0';
        string str = Console.ReadLine();
        cA = str[0];
        n++;
        nA++;
        for (int i = 1; i < 4; i++) {
            if (str[i] != cA) {
                if (n == 1) {
                    cB = str[i];
                    n++;
                    nB++;
                } else {
                    if (str[i] == cB) {
                        nB++;
                    } else {
                        Console.WriteLine("No");
                        return;
                    }
                }
            } else {
                nA++;
            }
        }
        if (n == 2 && nA == 2 && nB == 2) Console.WriteLine("Yes");
        else Console.WriteLine("No");
    }
}