<?php

$starting_numbers = explode(',', file_get_contents('input.txt'));

$numbers = array_map('intval', $starting_numbers);
$number = array_pop($numbers);
for ($i = count($starting_numbers); $i <= 2020; $i++) {
    $number = newNumber($number);
}

echo $number;

function newNumber(int $number = 0)
{
    global $numbers;
    if (!in_array($number, $numbers, true)) {
        $numbers[] = $number;

        return $number;
    }

    $indexes = array_keys($numbers, $number);

    if (count($indexes) === 1) {
        $numbers[] = 0;
        return 0;
    }

    $new_number = $indexes[count($indexes) - 1] - $indexes[count($indexes) - 2];
    $numbers[] = $new_number;

    return $new_number;
}