function principal_calc(){
	var principal = document.getElementById('salesprice').value;

	var interest  = document.getElementById('interest').value;
	
	var noy = document.getElementById('term').value;
	var down = document.getElementById('down').value;
	
	var msg;
	msg="";
	
	
	if (principal == '') { principal = '0'; }
	if (interest == '')  { interest = '0'; }
	if (noy == '') 		 { noy = '0'; }
	if (down == '') { down = '0'; }
	
	if (principal == "0"){ 
		var display = 'Please fill "Loan Amount" field.'; 
		msg += display +"\n"; 
		}
		
	if (interest == "0"){ 
		var display = 'Please fill "Interest Rate" field.'; 
		msg += display + "\n"; 
		}
	
	if (noy == "0"){ 
		var display = 'Please fill "Length of Loan" field.'; 
		msg += display + "\n"; 
		}
		
	if (principal != "0" && interest != "0" && noy != "0" ){ 
		var str = principal; 
		var a = (str.replace(/,/, "")); 
		var b = (down.replace(/,/, "")); 
		var amt = a - b; 
		var t_years = eval(noy*12); 
		var t_interest = eval(interest/1200); 
		var t = eval(1.0 /(Math.pow((1+t_interest),t_years))); 
			
			if(t < 1){ 
				var payment = eval((amt*t_interest)/(1-t)); 
			} else { 
				var payment = eval(amt/$t_years); 
			} 
			
		var total = payment.toFixed(2); 
		document.getElementById('mpay').value=total;
		
		
	}
	if (msg != "") { alert(msg); }
	return false;
	
}