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
$ant_arr = trim(fgets(STDIN));

$count1 = mb_substr_count($ant_arr, $ant_arr[0]);
$count2 = mb_substr_count($ant_arr, $ant_arr[1]);
$count3 = mb_substr_count($ant_arr, $ant_arr[2]);
$count4 = mb_substr_count($ant_arr, $ant_arr[3]);

if ($count1 === $count2 && $count4 === $count3) echo "Yes\n";
else echo "No\n";