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>

int main(void){
        char a[4];
        scanf("%s", a);
        int count;
        int sum =0;

        for(int i=0;i<4;i++){
                for(int j=0;j<4;j++){
                        if(a[i] == a[j]){
                                sum++;
                        }

                }
        }
        if(sum == 8){
                printf("Yes\n");
        }else{
                printf("No\n");
        }
        return 0;
}