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>

#define LENGTH 5

int main(void) {
    char a[LENGTH];

    scanf("%s", a);
    
    if((a[0] == a[1]) && (a[2] == a[3]) && (a[1] != a[2])){
        printf("Yes\n");
    }else if((a[1] == a[2]) && (a[0] == a[3]) && (a[0] != a[1])){
        printf("Yes\n");
    }else if((a[0] == a[2]) && (a[1] == a[3]) && (a[0] != a[1])){
        printf("Yes\n");
    }else{
        printf("No\n");
    }

    return 0;
}