function increment() {
    var passengers = document.getElementById("passengers");
    var value = parseInt(passengers.value, 10);
    if (value < 50) {
        passengers.value = value + 1;
    }
}

function decrement() {
    var passengers = document.getElementById("passengers");
    var value = parseInt(passengers.value, 10);
    if (value > 1) {
        passengers.value = value - 1;
    }
}