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 (GCC 5.4.1) (judge verdict: Accepted)

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

int main(void){
    int i,j,k;
    char S[10];
    bool flag = false;
    
    scanf("%s",S);
    
    for(i = 0;i < 4;i++){
        flag = false;
        for(j = 0;j < 4;j++){
            if(S[i] == S[j] && i != j){
                if(flag){
                    printf("No\n");
                    return 0;
                }
                flag = true;
            }
        }
        if(flag == false){
            printf("No\n");
            return 0;
        }
    }

    
    printf("Yes\n");
    return 0;
}