// JavaScript Document
function calc(myform) {
var p = eval(myform.p.value);
if (p == '') {alert('Instalment must be entered.');myform.p.focus();return false;}
if (p != parseFloat(p) || p < 1)
{alert('Instalment invalid (must be greater than zero).');myform.p.focus();return false;}
var r = eval(myform.r.value) / 100;
if (r == '') {alert('Interest Rate must be entered.');myform.r.focus();return false;}
if (r != parseFloat(r) || r < 0 || r > 1)
{alert('Interest rate invalid (must be between 0 and 100).');myform.p.focus();return false;}
var m=0;
for (var i=0; i< myform.m.length;i++) {
     if (myform.m[i].checked) m = myform.m[i].value;
}
if (m == 0) {alert('Instalment period must be selected.');return false;}
var y = eval(myform.y.value);
if (y == '') {alert('Loan period must be entered.');myform.y.focus();return false;}
if (y != parseFloat(y) || y < 0.25 || y > 50)
{alert('Loan period invalid (must be between 1/4 and 50 years).');myform.p.focus();return false;}
var i = r/m;
var n = y*m;
var a = Math.floor(p * ((1-Math.pow((1+i),-n))/i));
myform.a.value = a;
return false;
}