This is my first javascript program. I've been studying for a few days and thought I'd put it into practice.

The problem is I'm trying to grab 2 numbers and multiply them. This will be a program to calculate percentage of something. I'm testing if two numbers can be multiplied and the output inserted into a span tag with id="total". When I hit calculate, it prints the total in span tag but quickly refreshes with default value over the total value.

HTML
Code:
    	<form method="get" action="">
    		<fieldset>
    			<legend>Find Percentage of a Number</legend>
			    <ul>
			    	<li>
				    	<label for="base">Base:</label>
				    	<input type="text" id="base" />
				    </li>
			    	<li>
						<label for="percent">Percent:</label>
						<input type="text" id="percent" />
			    	</li>
			    	<li>
						<p><span id="total">Total?</span></p>
			    	</li>
			    	<li>
						<input type="submit" id="submit" value="Caculate?" onclick="mainInit()" />
			    	</li>
			    </ul>
		    </fieldset>
		    <script type="text/javascript" src="mathshop.js"></script>
    	</form>
JavaScript
Code:
function mainInit() {
	if(document.getElementById) {
		if(document.getElementById("submit").onclick) {
			grabValues();
		}
	}
	else {
		alert("Sorry, your browser does not support JavaScript.");
	}
}

function grabValues() {
	var a = document.getElementById("base").value;
	var b = document.getElementById("percent").value;
	findTotal(a, b);
}

function findTotal(base, percent) {
	var total = base * percent;
	document.getElementById("total").innerHTML = total;

}