Error NaN when trying to add two input fields in Javascript

I am trying to sum two text fields using Javascript and my result keeps coming back as NaN (not a number?) and I am entering only numbers in the input fields. Despite some debugging in Visual Studio, I still can't figure out why?

Here is my HTML code:

<div id="calc">
        <p>
            <label for="num1">Enter Your First Number:</label>
            <input type="text" name="txtNum1" id="txtNum1" size="5" />
        </p>

        <p>
            <label for="num1">Enter Your Second Number:</label>
            <input type="text" name="txtNum2" id="txtNum2" size="5" />
        </p>

        <input type="button" name="add" id="add" value="Add" onclick="addNum();" />
       

    </div>

        <div id="ShowResults"></div>

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And here is my javascript:

var firstNumber = parseInt(document.getElementById('txtNum1').value);
var secondNumber = parseInt(document.getElementById('txtNum2').value);

function addNum() {

        var result = firstNumber + secondNumber;

            resultMessage = "Your answer is: " + result;
            ShowResults.innerHTML = resultMessage;

}

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Any help would be appreciated!