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 PHP (5.6.30) (judge verdict: Runtime Error)

<?php
  fscanf(STDIN, "%d", $S);
  $arr = str_split($S);  
  if($arr[0] === $arr[1] === $arr[2] === $arr[3]){
              echo "No";
            exit;
  }
  elseif ($arr[0] === $arr[1] && $arr[2] === $arr[3]){
            echo "Yes";
            exit;
  }
//abab
  elseif($arr[0] === $arr[2] && $arr[1] === $arr[3]){
            echo "Yes";
            exit;
  }
//abba
  elseif($arr[0] === $arr[3] && $arr[1] === $arr[2]){
            echo "Yes";
            exit;
  }
   else{
                 echo "No";
   }