<script language="javascript">
                function addNumbers()
                {
                    var val1 = parseInt(document.getElementById('value1')[document.getElementById('value1').selectedIndex].innerHTML);
                        //var val1 = parseInt(document.getElementById("value1").value);
                        var val2 = 0;
                        if (document.getElementById('value2')){
                            val2 = parseInt(document.getElementById('value2')[document.getElementById('value2').selectedIndex].innerHTML);
                    }
                        if (val1 < 0) { val1 = 0; }
                        if (val2 < 0) { val2 = 0; }

            function checkbox () {

            if(ck1.checked == 1){ val1 =0;}
            if(ck2.checked == 1){ val2 =0;}


            }

                        var qty = document.getElementById("quantity");
                        if (val2 == 0) {
                            qty.value = val1;
                    } else {
                        qty.value = val1 + val2;
                    }
                }

        </script>

<input type=checkbox name=ck1 onChange="checkbox(ck1)" />

<input type=checkbox name=ck2 onChange="checkbox(ck2)" />