I would like to add these amounts using amount_t attribute. and display in a textbox

I have list of check boxes with certain amounts.

<input type="checkbox" name="subject1" value="course_id" amount_t="amount"  class="amount" />
<input type="checkbox" name="subject2" value="course_id" amount_t="amount"  class="amount" />
<input type="checkbox" name="subject3" value="course_id" amount_t="amount"  class="amount" />
<input type="checkbox" name="subject4" value="course_id" amount_t="amount"  class="amount" />


$(document).ready(function(){
        $(".amount").change(function(){
            if (($(this).is(':checked'))) {
            var res;
            var val1 = parseInt($(this).attr("amount_t"));
            var val2 = $("#amount").attr("value");
            console.log('value1',val1);
            console.log('value2',val2);
    res = val2.slice(-4);
    res = parseInt(res);
    var val3 = 'Amount : $ ';
    var val4 = val1 + res;
    var val =  val3 + val4;
    $("#amount").val(val);
}
else{
            var res;
            var val1 = parseInt($(this).attr("amount_t"));
            var val2 = $("#amount").attr("value");
            console.log('value1',val1);
            console.log('value2',val2);
    res = val2.slice(-4);
    res = parseInt(res);
    var val3 = 'Amount : $ ';
    var val4 = res - val1;
    var val =  val3 + val4;
    $("#amount").val(val);
        }
        });

});

Amount should not able to change by user.

it is is working fine for first selection, after that it is showing like initial. can you please help me.