function calculate(form) 
{
  // validate fields were completed
  if ( (form.principal.value == null || form.principal.value.length == 0) )
  {
  	alert("Please fill in the Purchase Price");
  	return;
  }else if	( (form.interest.value == null || form.interest.value.length == 0) )
  {
  	alert("Please fill in the Loan Interest Rate");
  	return;
  }else if	( (form.payments.value == null || form.payments.value.length == 0) )
  {
  	alert("Please fill in the Mortgage Term");
  	return;
  }

 	// find total number of payments
	var total_payments = form.payments.value * 12;
	// get interest rate
	var interest_rate = form.interest.value;
	if (interest_rate == 0) { 
		interest_rate = .001;
	}else if (interest_rate > 1.0) {
  	interest_rate = interest_rate / 100.0;
  }
	// get pricipal amount
	var principal_amount = getPrincipalAmount(form.principal.value);

	// Calculate payments!
	interest_rate /= 12;
	var power = 1;
	for (var x = 0; x < total_payments; x++){
		power = power * (1 + interest_rate);
	}
	var monthly_payment = (principal_amount * power * interest_rate) / (power - 1);
	document.all.payment.innerHTML = "Your Monthly Payment is $"+parseInt(monthly_payment);
}

function getPrincipalAmount( principal) {
 var comma = ",";
 var blank = "";
 var output = "";

 for( var x=0; x<principal.length; x++ ) {
  if(principal.substring(x,x+1) == comma)
   output += blank;
  else
   output += principal.substring(x,x+1)
 }
 return output;
}