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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php
  ini_set('error_reporting', E_ALL & ~E_NOTICE);

  fscanf(STDIN, "%s", $S);
  $sum = [];

  $a = str_split($S);
  for ($i = 0; $i < count($a); $i++) {
    $sum[$a[$i]]++;
  }
  $vals = array_values($sum);

  $ans = "No";
  if (count($vals) === 2) {
    if ($vals[0] === 2 && $vals[1] === 2) {
        $ans = "Yes";
    }
  }

  echo $ans;