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 in[5];
	scanf("%s", in);
	char a = in[0];
	char b = 0;
	for(int i = 0; i < 4; i++){
		if(a != in[i]){
			b = in[i];
			break;
		}
	}
	if(b == 0){
		printf("No\n");
		return 0;
	}
	int c1 = 0, c2 = 0;
	for(int i = 0; i < 4; i++){
		if(a == in[i])c1++;
		if(b == in[i])c2++;
	}
	if(c1 == 2 && c2 == 2)
		printf("Yes\n");
	else
		printf("No\n");
	return 0;
}