document.querySelector('#a_amount').addEventListener('keyup', calculate) ;
  document.querySelector('#d_payment').addEventListener('keyup', calculate) ;
  document.querySelector('#r_amount').addEventListener('keyup', calculate) ;
  document.querySelector('#p_period').addEventListener('change', calculate) ;
  document.querySelector('#interest_rate').addEventListener('change', calculate) ;

  if (document.readyState === 'complete') {
   document.getElementById('interest_rate').value = document.getElementById('default_interest_rate').textContent;
  }

  function calculate(e) {
   var downPayment = document.getElementById('d_payment').value;
   var autoAmount = document.getElementById('a_amount').value;
   var paymentPeriod = document.getElementById('p_period').value;


   var iRateValue = jQuery('.i_rate_value').text();
   var iRateValueNew = parseFloat(iRateValue);

   var amort = document.getElementById('p_period').value*12;
   var rate = (iRateValueNew/12)/100;
   var rate2 = 1+rate;
   var rate3 = Math.pow(rate2,amort);
   var rate4 = rate3-1;
   var rate5 = rate/rate4;
   var rate6 = rate + rate5;
   var prin = autoAmount - downPayment;
   var monthlyPayment = Math.floor(rate6*prin)

   if(isNaN(monthlyPayment)){ monthlyPayment = 0; }

   // Computation: http://www.javascriptkit.com/script/cut155.shtml
   var princ = autoAmount - downPayment;
   var term  = paymentPeriod;
   var intr   = document.getElementById('interest_rate').value / 1200;
   //document.calc.pay.value = princ * intr / (1 - (Math.pow(1/(1 + intr), term)));


   document.getElementById('r_amount').textContent = Math.round( princ * intr / (1 - (Math.pow(1/(1 + intr), term))) );
   
  }