function numberOfDiscIntersections(array $a): int
{
    $sum   = $active = 0;
    $c     = count($a);
    $start = $end = array_fill(0, $c, 0);

    foreach ($a as $k => $v) {
        $key = $k < $v ? 0 : $k - $v;
        $start[$key]++;

        $key = $k + $v >= $c ? $c - 1 : $k + $v;
        $end[$key]++;
    }

    foreach ($a as $k => $v) {
        $sum    += $active * $start[$k] + ($start[$k] * ($start[$k] - 1)) / 2;
        $active += $start[$k] - $end[$k];
        if ($sum > 10000000) {
            return -1;
        }
    }

    return $sum;
}