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: Runtime Error)

#include<stdio.h>
int main () {
	char S[5];
	char s[5];
	scanf("%s", &S[0]);
	int cnt = 1;
	s[0] = S[0];
	for(int i = 1; i < 4; ++i){
		for(int k = 0; k < cnt; k++){
			if(s[k] != S[i]) {
				cnt++;
				continue;
			}
		}
	}
	if(cnt == 2) puts("Yes");
	else puts("No");
	return 0;
}