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

void main(void){
	char buf[5]={'\0'};
	char ch;
	int cnt=0;
	char *pt=NULL;
	int ans=0;

	scanf("%s",buf);

    cnt=0;
	while(1){
	  if(buf[cnt+1]=='\0'){
		  break;
	  }
	  if(buf[cnt]!='\n'){
		ch=buf[cnt];
		buf[cnt]='\n';
		pt=strchr(buf,ch);
		if(pt==NULL){
			ans=-1;
			break;
		}else{
			*pt='\n';
			pt=NULL;
		}
	  }

	cnt++;
  }
  if(ans==0){
	  printf("YES\n");
  }else{
	  printf("NO\n");
  }

}