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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
#include <string>
#include <vector>
#include <iomanip>
#include <algorithm>
#include <queue>
#include <bitset>
#include <map>
#include <math.h>
#include <utility>

using namespace std;

int main(){

    string S;
    cin >> S;

    vector<int> count(4);

    for (int i = 0; i < 4; i++){
        for (int j = i+1; j < 4; j++){
            if (S[i] == S[j]){
                count[i]++;
                count[j]++;
            }
        }
    }

    bool pair = true;
    for (int i = 0; i < 4; i++){
        if (count[i] != 1) pair = false;
    }

    if (pair) cout << "Yes" << endl;
    else cout << "No" << endl;
    
    return 0;
}