function PVIFA(annual_rate, years, period){
    var monthly_rate=annual_rate/period;
    var factor=0;
    var base_rate=1+monthly_rate;
    var denominator=base_rate;
    for (var i=0;i<(years*period);i++){
        factor+=(1/denominator);
        denominator*=base_rate;}
		return factor;
}
    
function calculate(form) {
	income = form.income.value;
	debt = form.debt.value;
	down = form.down.value;
	interest = form.interest.value;
	interest = interest/100;
	years = form.length.value;
	monthly_factor=PVIFA(interest, years, 12);
	maxmonth = Math.round((income/12)*.32);
	maxmonth = maxmonth - debt;
	price = (maxmonth*monthly_factor) + parseFloat(down);
	price = Math.round(price);
	form.maxmonth.value=maxmonth;
	form.maxprice.value=price;
}