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 (Clang 3.8.0) (judge verdict: Wrong Answer)

#include<stdio.h>
int main(){
  int a;
  int b[21];
  int c = 0;
  int i;
  scanf("%d",&a);
  for(i=0;i<=a-1;i++){
    scanf("%d", &b[i]);
  }
  for(i = 0; i <= a-2;i++){
    if((b[i]<b[i+1]) && (b[i+1]<b[i+2])){
      c++;
    }else if((b[i]>b[i+1]) && (b[i+1]>b[i+2])){
      c++;
    }else{
      c += 0;
    }
  }
  printf("%d", c);
  return 0;
}