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(void){
	
  int n;
  scanf("%d",&n);
  
  int line[n];
  int i;
  int cnt = 0;
  
  for(i = 0 ; i < n ; i++){
      scanf("%d",&line[i]);
  }
  
  for(i = 1 ; i < n - 1 ; i++){
      if(line[i+i] > line[i] &&line[i] > line[i-1]){
          cnt++;
      }else if(line[i+i] < line[i] &&line[i] < line[i-1]){
          cnt++;
      }
  }
  
  printf("%d",cnt);
  
  
  return 0;
}