function calculate_savings(){
    var cigs_per_day = document.getElementById('cigs_per_day').getElementsByTagName('input')[0].value;
    var price_per_pack = document.getElementById('price_per_pack').getElementsByTagName('input')[0].value;

    price_per_pack = price_per_pack.replace(/\$/, "")

    var saveperday = cigs_per_day * price_per_pack / 20;
    var saveperweek = saveperday * 7;
    var savepermonth = saveperday * 30;
    var saveperyear = saveperday * 365;
    var saveper5year = saveperday * 365 * 5;
    var saveper10year = saveperday * 3650;
    document.getElementById('weekly').innerHTML = '$' + addCommas(saveperweek.toFixed(2));
    document.getElementById('monthly').innerHTML = '$' + addCommas(savepermonth.toFixed(2));
    document.getElementById('yearly').innerHTML = '$' + addCommas(saveperyear.toFixed(2));
    document.getElementById('year5').innerHTML = '$' + addCommas(saveper5year.toFixed(2));
    document.getElementById('year10').innerHTML = '$' + addCommas(saveper10year.toFixed(2));

    document.getElementById('price_per_pack').getElementsByTagName('input')[0].value = "$" + price_per_pack;
}

function addCommas(nStr)
{
    nStr += '';
    x = nStr.split('.');
    x1 = x[0];
    x2 = x.length > 1 ? '.' + x[1] : '';
    var rgx = /(\d+)(\d{3})/;
    while (rgx.test(x1)) {
        x1 = x1.replace(rgx, '$1' + ',' + '$2');
    }
    return x1 + x2;
}