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: Wrong Answer)

#include<stdio.h>
int main()
{
	char S[4];
	int a=0;
	int b=0;
	char c;
	char d;
	int OK;
	scanf("%s",S);
	c = S[0];
	for(int i=0;i<4;i++){if(S[i]!=c){d=S[i];OK=1;}}
	for(int i=0;i<4;i++){if(S[i]==c)a++;}
	if(OK){
		for(int i=0;i<4;i++){if(S[i]==d)b++;}
	}else
	{
		b=0;
	}
	printf("%s\n",(a==2 && b==2)? "Yes":"NO");
	return 0;
}