Onclick increase or decrease value in jQuery

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<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>

<script type="text/javascript">
$(function()
{
    $(".add").click(function()
    {
        var currentVal = parseInt($(this).next(".qty").val());
        if (currentVal != NaN)
        {
            $(this).next(".qty").val(currentVal + 1);
        }
    });

    $(".minus").click(function()
    {
        var currentVal = parseInt($(this).prev(".qty").val());
        if (currentVal != NaN)
        {
            if(currentVal > 0){
                    $(this).prev(".qty").val(currentVal - 1);
                }

        }
    });
});

</script>



<form id="myform">
        <input type="button" value="+" id="add1" class="add" />       
        <input type="text" id="qty1" value="0" class="qty" />       
        <input type="button" value="-" id="minus1" class="minus" /><br /><br />
    </form>

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