function calculatePayment() {
      
      var p = document.getElementById("principal").value;
      var n = document.getElementById("months").value;
      var i = document.getElementById("rate").value;

      if (isNaN(p) || p.length == 0) {
        alert("Please enter a valid number for principal amount.");
        return;
      }
      if (isNaN(n) || n.length == 0) {
        alert("Please enter a valid number for number of months.");
        return;
      }
      if (isNaN(i) || i.length == 0) {
        alert("Please enter a valid number for interest rate.");
        return;
      }

      var a = (1 + ((i/100)/12));
      var M = p * (a-1) / (1 - Math.pow(a, -1 * n));

      document.getElementById("payment").value = toDollarsAndCents(M);
    }

    function toDollarsAndCents(n) {
      var s = "" + Math.round(n * 100) / 100;
      var i = s.indexOf('.');
      if (i < 0) return s + ".00";
      var t = s.substring(0, i + 1) + s.substring(i + 1, i + 3);
      if (i + 2 == s.length) t += "0";
      return t;
    }