Adding Forms But It's Limited To Text Input Forms?

how to add forms in javascript, but it's limited to text input forms.

<script type="text/javascript"><!--
function updatesum() {
document.form.sum.value = (document.form.sum1.value -0) + (document.form.sum2.value -0);
}
//--></script>

Where the inputs sum1 and sum2 are text fields you put whatever numbers you want in. That works fine. Great. Now what I'm having trouble with is modifying the code so that it will add one form with an input number with a form that spits out a randomly generated number.

<input type="button" value="D20" onclick="this.form.display.value = Math.round (20 * Math.random())" class="buttonHi" />  <input name="display" type="text" size="6" value="" />

This is what I'm using for my random number generator. So basically I want to be able to put, say, 5, into the input text field above this. And then click on the d20 button to get a random number, say, 15, and then have the first code add the inputted 5 with the randomly generated 15.