function flatlandSpaceStations($n, $m, $c) {
    sort($c);

    $max = $c[0];
    for($i = 0; $i < $m - 1; $i++) {
        $dist = floor(($c[$i + 1] - $c[$i]) / 2);
        if($dist > $max) {
            $max = $dist;
        }
    }
    
    $max = max($max, $n - 1 - $c[$m - 1]);
    
    return $max;
}