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()
{
	char input[256];
	scanf("%255s", input);
	char onec = 0;
	char twoc = 0;
	int onen = 0;
	int twon = 0;

	for( int i=0; i < 4; i++ ) {
		if( input[i]==onec ) {
			onen++;
		} else if( input[i]==twoc ) {
			twon++;
		} else if( onec == 0 ) {
			onec = input[i];
			onen++;
		} else if( twoc == 0 ) {
			twoc = input[i];
			twon++;
		}
	}

	if( onen==2 && twon==2 ) {
		printf("Yes\n");
	} else {
		printf("No\n");
	}

	return 0;

}