<?php

$list_1 = [];
$list_2 = [];

$lines = array_filter(explode("\n", file_get_contents(__DIR__.'/input.txt')), static fn($line) =>!empty($line));

foreach ($lines as $line) {
    [$a, $b] = explode("   ", $line);
    $list_1[] = $a;
    $list_2[] = $b;
}

$similarity_score = 0;

$count_list_2 = array_count_values($list_2);
foreach ($list_1 as $item) {
    $similarity_score += array_key_exists($item, $count_list_2) ? $count_list_2[$item] * $item : 0;
}

echo $similarity_score;