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];
    int cnt[4] = {0};
    scanf("%s", s);
    for (int i = 0; i < 3; i++)
        for (int j = i + 1; j < 4; j++)
            if (s[i] == s[j])
            {
                cnt[i]++;
                cnt[j]++;
            }
            
    for (int i = 0; i < 3; i++)
        if (cnt[i] != cnt[i + 1] || cnt[i] != 1 || cnt[3] != 1)
        {
            puts("No");
            return 0;
        }
    puts("Yes");
    return 0;
}