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 <string.h>

#define LENGTH 256

int main(void) {
    char input[LENGTH];
    int r, i, cmp;
    int count1 = 0;
    int count2 = 0;
    char n[4];
    fgets(input, LENGTH-1, stdin);
    sscanf(input, "%s", n);

    for(i = 0; i < 4; i++){
        for(r = i + 1; r < 4; r++){
            if(n[i] == n[r]){
                if(count1 == 0 && count2 == 0){
                    count1++;
                }else{
                    count2++;
                } 
            }
        }
    }
    if(count1 == 1 && count2 ==1){
        printf("%s\n","Yes");
    }else{
        printf("%s\n","No");
    }
    return 0;
}