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 a[5];
  int count[26]={0};
  int i,j,flag=0;
  scanf("%s",a);
  
  for(i=0;i<4;i++){
  	for(j=0;j<26;j++){
    	if(a[i]=='A'+j) count[j]++;
    }
  }
  
  for(j=0;j<26;j++){
    //printf("count[%d]=%d\n",j,count[j]);
    if(count[j]==0 || count[j]==2) flag =0;
    else{
      flag=1;
      break;
    }
  }
  
  if(flag == 1) printf("No\n");
  else printf("Yes\n");
  
  return 0;
  
}