<?php

$numbers = array_map('intval', explode("\n", file_get_contents('input.txt')));

foreach ($numbers as $number) {
    $number_to_2020 = 2020 - $number;
    if (in_array($number_to_2020, $numbers, true)) {
        echo $number_to_2020 * $number;
        exit;
    }
}