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 <stdio.h>
#include <stdlib.h>
#include <cmath>

int main() {

    int S, N, L, tmp1;
    std::string s;
    int ans = 0;

    std::cin >> s;

    int flag1 = 1;
    int flag2 = 0;
    if(s[0] == s[1]){
        flag1++;
    }else {
        flag2++;
    }

    if(s[0] == s[2]){
        flag1++;
    }
    else {
        if(s[1] == s[2]){
            flag2++;
        }else{
            if(flag2 == 0){
                flag2++;
            }
            else {
                std::cout << "No" << std::endl;
                return 0;
            }
        }
    }
    if(s[0] == s[3]){
        flag1++;
    }
    else {
        if(s[1]==s[3]){
            flag2++;
        }
        else {
            if(flag2 == 1 && s[2] == s[3]){
                std::cout << "Yes" << std::endl;
                return 0;
            }
            else {
                std::cout << "No" << std::endl;
                return 0;
            }
        }
    }
    if(flag1 > 2 || flag2 > 2){
        std::cout << "No" << std::endl;
        return 0;
    }

    std::cout << "Yes" << std::endl;
    return 0;
}