var rp = 0.07;

	//checks that data are valid 
	function checkNumber(input, min, max, msg) {

		msg = msg + " has invalid data: ";

		//this makes sure that the number is a number
		var str = input.value;
		for (var i = 0; i < str.length; i++) {
			var ch = str.substring( i, i + 1)
			if ((ch < "0" || "9" < ch) && ch != '.') {
				alert(msg);
				return false;
			}
		}

		//this makes sure that the number lies between the min and max values allowed
		var num = 0 + str
		if (num < min || max < num) {
			alert(msg + " not in range [" + min + ".." + max + "]");
			return false;
		}
		input.value = str;
		return true;
	}

	function calcVal(e) {
		if (e.value != null && e.value.length != 0)
		{
			e.value = "" + eval(e.value);
		}
		calcForm(e.form);
	}

	function calcForm(f) {
		var b = f.borrow.value;
		var y = f.years.value;
		var r = f.rate.value;

		//making sure that an entry has been made in each field.
		if (b == "" || r == "") {
			//alert('not all fields filled in');
			return;
		}

		// making sure that entries are valid by using check number
		if (!checkNumber(f.borrow, 1, 99999999, "Amount") ||
			!checkNumber(f.rate, .001, 1000, "Interest Rate") ||
			!checkNumber(f.years, 5, 30, "Period")) 
		{
			f.Cm.value = "Invalid";
			return;
		}

		r = r / 100;
		var p = ((b*r)/12) * (1/(1-(Math.pow(1/(1+r),y))));
		f.Cm.value = poundsPence( p );
		p = ((b*rp)/12) * (1 / (1-(Math.pow((1/(rp + 1)),y))));
		f.CCm.value = poundsPence( p );
		p = (b*r)/12;
		f.CI.value = poundsPence( p );
		p = (b*rp)/12;
		f.CCI.value = poundsPence( p );
	}

	function poundsPence( N ) {
		// don't try this with ie3 because it's rubbish
		if ((navigator.appName.indexOf('Microsoft')>-1)
			&& (navigator.appVersion.indexOf('3.0')>-1) )
		{
			return N;
		}
		var S = new String( N );
		var i = S.indexOf('.');
		if (i != -1) {
			S = S.substr( 0, i+3 );
			if (S.length-i < 3)
				S = S + '0';
		}
		return S;
	}