Trouble with Js/Html - Checkbox integer values for 'combat calculator' for game

Okay - I have been scouring for an answer to why my code isn't working.. and I just can't find it. I am assuming it comes from my checkbox value not returning as an integer, but I don't know how exactly to fix it..

http://ift.tt/17XgiBQ

<script type="text/javascript">
function update_value(chk_bx){
    if(chk_bx.checked)
    {
        var a=-1;}
    else{
        var a=0;
    }
    document.getElementById('result').value =a;

}

Checkbox

Ignore most of my awful code (or give advice in general, if you're crazy awesome..) but take a look at how the calculator works, and you should quickly see that the 'archer field' doesn't work at all - this is due to it being my tester field and yeah, lemme direct you to the 'trouble code' spots..

See lines: 23,30, and 279-292

I want the values from the checked or unchecked box to return as integers, and then be applied to the calculator. An example of what it should do when FINISHED - Forest provides cover for units against archers, so if the FOREST box is checked, the integer would return -1 and apply a -1 to all archer units.

Please assist if you have the patience (complete code noob.. bet you noticed..) or point me in the right direction..