function round(number)
{
  return Math.round(number*Math.pow(10,2))/Math.pow(10,2);
}

function calculateMortgagePayment()
{
	var loan_interest = $F('loan_interest');
	var loan_amount = $F('loan_amount');
	var loan_duration = $F('loan_duration');
	var annual_tax = $F('annual_tax');
	var annual_insurance = $F('annual_insurance');

	var monthly_interest = loan_interest / 1200;
	var monthly_taxes = round(annual_tax / 12);
	var monthly_insurance = round(annual_insurance / 12);

	var counter = 1;  
	var mcounter = 1 + monthly_interest;  
	
	for (i = 0; i < loan_duration * 12; i++)  
	{   
		counter = counter * mcounter; 
	}
	
	var base_monthly_payment = round(loan_amount * monthly_interest / (1 - (1 / counter)));
	var monthly_payment = round( base_monthly_payment + monthly_taxes + monthly_insurance );

	if (isNaN(monthly_payment))
	{
		$('monthly_payment_amount').innerHTML = '<span class="error">Error:<br />Please enter numbers for all values.</span>';
		if (isNaN(loan_interest))
		{
			$('loan_interest').focus();				
			$('loan_interest').select();				
		}
		if (isNaN(loan_amount))
		{
			$('loan_amount').focus();				
			$('loan_amount').select();				
		}
		if (isNaN(annual_tax))
		{
			$('annual_tax').focus();				
			$('annual_tax').select();				
		}
		if (isNaN(annual_insurance))
		{
			$('annual_insurance').focus();				
			$('annual_insurance').select();				
		}
	}
	else
	{
		$('monthly_payment_amount').innerHTML = 'Your Monthly Payment*:<br />$' + monthly_payment;
	}
}