<?php

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

sort($numbers);

$difference_of_1 = 0;
$difference_of_3 = 0;

$current_jolt = 0;

while($current_jolt < max($numbers)) {
    if (in_array($current_jolt + 1, $numbers, true)) {
        $current_jolt++;
        $difference_of_1++;
        continue;
    }
    if (in_array($current_jolt + 3, $numbers, true)) {
        $current_jolt += 3;
        $difference_of_3++;
        continue;
    }
}

// your device's built-in adapter is always 3 higher than the highest adapter
$difference_of_3++;
$current_jolt += 3;

echo $difference_of_1 * $difference_of_3;