var sum = 0;
var Discount = 0;
var afterDiscount = 0;
var price;

while (true) {
    price = parseInt(prompt("Please enter a Price or -1 to STOP"));
    if (price === -1){
    break;
    }
    if (price <= 50) {
            alert(price);
    } else if (price > 50 && price <= 100) {
            alert(price-price*.10);
    } else if (price > 100) {
            alert(price-price*.20);
    }
    sum += price;
}
alert(sum);