<?php

$intervals = array_map(fn($interval) => explode('-', $interval), explode(',', file_get_contents('./input.txt')));

$invalids = [];

foreach ($intervals as $interval) {
    [$start, $end] = $interval;
    $current = $start;
    while ($current <= $end) {
        if (is_invalid($current)) {
            $invalids[] = $current;
        }

        $current++;
    }
}

function is_invalid($current): bool {
    if (strlen($current) % 2 !== 0) {
        return false;
    }

    $middle = strlen($current) / 2;
    return substr($current, 0, $middle) === substr($current, $middle);
}

echo array_sum($invalids);