<?php

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

$tmp_numbers = array_map('intval', $starting_numbers);
$count_numbers = [];
foreach ($tmp_numbers as $i => $tmp_number) {
    if (array_key_exists($tmp_number, $count_numbers)) {
        $count_numbers[$tmp_number][] = $i;
    } else {
        $count_numbers[$tmp_number] = [$i];
    }
}
$number = end($tmp_numbers);
unset($tmp_numbers, $tmp_number);

for ($i = count($starting_numbers); $i <= 29999999; $i++) {
    $number = newNumber($i, $number);
}

echo $number;

function newNumber(int $i, int $number = 0)
{
    global $count_numbers;
    if (!array_key_exists($number, $count_numbers)) {
        $count_numbers[$number] = [$i];

        return $number;
    }

    $count_indexes = count($count_numbers[$number]);

    $new_number = $count_indexes === 1 ? 0 : $count_numbers[$number][$count_indexes - 1] - $count_numbers[$number][$count_indexes - 2];

    $count_numbers[$new_number][] = $i;

    return $new_number;
}