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>
#include<stdlib.h>

int main(void) {
  char s[4];
  char a, b;
  scanf("%s", s);
  a = s[0];
  if(s[1] == a) {
    if (s[2] == s[3] && s[2] != a) {
      printf("Yes\n");
      return 0;
    }
    else {
      printf("No\n");
      return 0;
    }
  }
  else {
    b = s[1];
    if(s[2] == a) {
      if(s[3] == b) {
        printf("Yes\n");
        return 0;
      }
      else {
        printf("No\n");
        return 0;
      }
    }
    else if(s[2] == b) {
      if(s[3] == a) {
        printf("Yes\n");
        return 0;
      }
    }
    else {
      printf("No\n");
      return 0;
    }
  }
}