Milk Cost Calculator

function milkCost(givenMoney) {
    givenMoney = prompt("What is your budget?")
    var numberOfBottles = Math.floor(givenMoney/1.5);
    alert("You can buy: " + numberOfBottles + " bottles.")
    return givenMoney % 1.5;
}

var change = milkCost();
alert("Your change is: " + change + " USD.");