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 <stdlib.h>
#include <string.h>
#include <stdbool.h>

int main(int argc, void **arg)
{
    char s[5], str[3] = "  ";
    int n[2] = { 0, 0 };
    int i, j;

    scanf("%s", s);

    for (i = 0; i < 4; i++)
    {
        for (j = 0; j < 2; j++)
        {
            if( s[i] == str[j] ) {
                n[j]++;
                break;
            }
        }
        if( j != 2 ) continue;
        
        for (j = 0; j < 2; j++)
        {
            if( str[j] == ' ' ) {
                str[j] = s[i];
                break;
            }
        }
        if( j != 2 ) continue;

        puts("No");
        return 0;
    }

    if( str[1] == ' ' ) puts("No");
    else                puts("Yes");

    return 0;
}