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 s[5];
    scanf("%s",s);
    int flag = 0;
    for(int i = 0; i < 4; i++) {
        int count = 0;
        for(int j = i; j < 4; j++) {
            if(s[i] == s[j]) count++;
        }
        if(count == 2) flag++;
    }
    if(flag == 2) {
        printf("Yes\n");
    } else {
        printf("No\n");
    }
    return 0;
}