list($days, $cost) = explode(" ", trim(fgets(STDIN)));
$salary = 0;
$now_plase = "";

for ($i=1; $i<=$days; $i++) {

    list($tokyo, $kyoto) = explode(" ", trim(fgets(STDIN)));

    // 最初は東京に居る
    if ($i == 1) {

        $tokyo_salary = $tokyo;
        $kyoto_salary = $kyoto - $cost;

    } else {

        if ($now_plase == 'tokyo') {
            $tokyo_salary = $tokyo;
            $kyoto_salary = $kyoto - $cost;
        } elseif ($now_plase == 'kyoto') {
            $tokyo_salary = $tokyo - $cost;
            $kyoto_salary = $kyoto;
        }

    }

    $now_plase = ($tokyo_salary > $kyoto_salary) ? 'tokyo' : 'kyoto';

    if ($tokyo_salary > $kyoto_salary) {
        $salary = $salary + $tokyo_salary;
    } elseif ($kyoto_salary > $tokyo_salary)  {
        $salary = $salary + $kyoto_salary;
    }
    
}

echo $salary."\n";