Proper Use of Nan

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How would I implement NaN and have Inavlid input display in a

element
To do this I have to use isNaN to verify that numerical values are input.
I just want it to display "Invalid input" whenever a non numeric value is put in either of the two first text boxes.

http://jsfiddle.net/hq3m1uns/1/ this is my fiddle link

<!DOCTYPE html>
    <html lang="en">
    <head>
        <meta charset="utf-8">
        <title>Question2</title>
        <style> 
        
        </style>

        <script type="text/javascript">

    

window.onload = function(){
    
};


 function add_number(){



var first_number = document.querySelector("#tb1").value;
  var second_number = document.querySelector("#tb2").value;
  var First = parseInt(first_number);
  var Second = parseInt(second_number);
  var result = First + Second;
  document.getElementById('tb3').setAttribute("value",result);
}



        </script>


    </head>
    <body>        
        <div>
  <h1>Add two number using text box as input using javascript</h1>
</div>
Enter First Number : <br>
<input type="text" id="tb1" name="TextBox1">
<br> Enter Second Number : <br>
<input type="text" id="tb2" name="TextBox2">
<br> Result : <br>
<input style="width: 50%" type="text" id="tb3" name="TextBox3" >
<br>
<input onclick="add_number()" type="button" id="b1" value="GO" />


</body>
</html>

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