Add value to variable if checkbox is checked, restore previous value if not checked (jquery)
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I'm a noob with jquery (and for "noob" I mean "I began using jquery a few hours ago"). What I want to do is quite simple: add a value to a variable when a checkbox is checked, restore the previous value of the same variable if the checkbox is not checked. The value must be written inside a span of the document. I have two problems with the code I wrote:

1- The value is not restored when I uncheck the checkbox;

2- I want the value to be always shown in the document, even at the first load of the page and I simply don't know how to do this (now it appears after the first click, because it's written inside the if condition)

Here's the script: what's wrong with it?

    <script type="text/javascript">
    var pds1=0;

    $(document).ready(function() {
        $("#mat1").click(function() {
            if($('#mat1').is(':checked')) { 
            	pds1=pds1+10;
		$('#perk1').html(pds1);
            } else {
		if(pds1>0)
		{
            	pds1=pds1-10;
		}
		else {}
            }
        });
    });
    </script>

And here's the HTML:

    <input id="mat1" type="checkbox" /> <span id="perk1"></span>