关于onchange的问题，急求解决。

var ptype = document.getElementById("ptype");
    var pcount = document.getElementById("pcount");
    var con = document.getElementById("con");

    ptype.onchange = function()
    {
        var price = parseInt(ptype.options[ptype.selectedIndex].value);
        var count = parseInt(pcount.value);

        con.value = price * count;
    }