Kilograms to Pounds on focuse chang

HTML

<div>
    <label>Pounds (lb):
        <input id="pound" onkeyup="ptok()" />
    </label>
</div>
<div>&nbsp;</div>
<div>
    <label>Kilograms (kg):
        <input id="kg" onkeyup="ktop()" />
    </label>
</div>

JavaScript

function ptok() {
    var pound = parseFloat(document.getElementById('pound').value);
    var kg = "";
    if (!isNaN(pound)) kg = pound * 0.45359237;
    document.getElementById('kg').value = kg;
}

function ktop() {
    var kg = parseFloat(document.getElementById('kg').value);
    var pound = "";
    if (!isNaN(kg)) pound = kg * 2.20462262;
    document.getElementById('pound').value = pound;
}