function checkNumber(input, min, max, msg) {

		msg = "The " + msg + " field is invalid. ";

		//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 + "The figure should be between " + min + " and " + max + ".");
			return false;
		}
		input.value = str;
		return true;
	}

	function computeField(input) {
		if (input.value != null && input.value.length != 0)
		{
			input.value = "" + eval(input.value);
		}
		computeForm(input.form);
		//clearForm(input.form);
		return false;
	}

	function computeForm(form) {
		var A=form.A.value; // Principal amount
		var T=form.T.value; // Term - number of years
		var R=form.R.value; // Rate of interest

		//making sure that an entry has been made in each field.
		if ((A == null || A.length == 0) ||
			(R == null || R.length == 0)) 
		{
			//alert('not all fields filled in');
			return;
		}

		// making sure that entries are valid by using check number
		if (!checkNumber(form.A, 1, 99999999, "'Mortgage required'") ||
			!checkNumber(form.R, .001, 1000, "Interest rate") ||
			!checkNumber(form.T, 5, 40, "Repayment period")) 
		{
			form.Cm.value = "Invalid";
			return;
		}

		// maths et al to be computed
		R = R / 100;
		var P = ((A*R)/12) * (1/(1-(Math.pow(1/(1+R),T))));
		form.Cm.value = poundsPence( P );
		P = ((A*0.12)/12) * (1 / (1-(Math.pow((1/1.12),T))));
		//form.CCm.value = poundsPence( P );
		P = (A*R)/12;
		form.CI.value = poundsPence( P );
		P = ((A*0.12)/12);
		//form.CCI.value = poundsPence( P );
		return false;
	}

	function poundsPence( N ) {
		// won't work as intended in ie3
		if ((navigator.appName.indexOf('Microsoft')>-1)
			&& (navigator.appVersion.indexOf('3.0')>-1) )
		{
			return N;
		}
		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;
	}

	//clears form
	function clearForm(form) {
		
		form.A.value = "";
		form.T.value = "";
		form.R.value = "";
		
		form.Cm.value = "";
		form.CI.value = "";
		return false;
	}