Errulator

Gives the right answer for 1 + 1, but not for 387,726,473 + 543,747,869!
Embed the Errulator on YOUR website!

<div id="calc">
  <input type="text" id="calcscreen" readonly>
  <button class="calcbtn" onclick="append(1);">1</button>
  <button class="calcbtn" onclick="append(2);">2</button>
  <button class="calcbtn" onclick="append(3);">3</button>
  <button class="calcbtn" onclick="append(4);">4</button>
  <button class="calcbtn" onclick="append(5);">5</button>
  <button class="calcbtn" onclick="append(6);">6</button>
  <button class="calcbtn" onclick="append(7);">7</button>
  <button class="calcbtn" onclick="append(8);">8</button>
  <button class="calcbtn" onclick="append(9);">9</button>
  <button class="calcbtn" onclick="append(0);">0</button>
  <button class="calcbtn" onclick="append(' + ');">+</button>
  <button class="calcbtn" onclick="append(' - ');">-</button>
  <button class="calcbtn" onclick="append(' * ');">×</button>
  <button class="calcbtn" onclick="append(' / ');">÷</button>
  <button class="calcbtn" onclick="append('.');">.</button>
  <button class="calcbtn" onclick="append('(');">(</button>
  <button class="calcbtn" onclick="append(')');">)</button>
  <button class="calcbtn" onclick="calcBackspace();">←</button>
  <button class="calcbtn" onclick="calcClear();">C</button>
  <button class="calcbtn gogo-orange-btn" onclick="invalidateAnswer();">=</button>
</div>
<script>
  /*This code may not be distributed without express written/typed permission by Gopal Othayoth (cerebraldatabank@gmail.com).*/
  function append(appendchar) {
    document.getElementById("calcscreen").value += appendchar;
  }
  function calcDisplay(calcdisplaytext) {
    document.getElementById("calcscreen").value = calcdisplaytext;
  }
  function calcBackspace() {
    document.getElementById("calcscreen").value = document.getElementById("calcscreen").value.substring(0, document.getElementById("calcscreen").value.length - 1);
  }
  function calcClear() {
    document.getElementById("calcscreen").value = "";
  }
  function invalidateAnswer() {
    calcDisplay(eval(document.getElementById("calcscreen").value));
    if (document.getElementById("calcscreen").value.length > 3)
    var correctAnswer = document.getElementById("calcscreen").value;
    var futureMessedUpPart = correctAnswer.substring(0, correctAnswer.length - 3);
    var correctPart = correctAnswer.substr(3);
    if (futureMessedUpPart != "" || futureMessedUpPart != null || futureMessedUpPart != undefined) {
      var messedUpPart = futureMessedUpPart * 0.05;
      calcDisplay(messedUpPart + correctPart);
    } 
    else {
      calcDisplay(correctAnswer);
    }
  }
</script>