How to use javascript to get html special characters in text input

I want to sent some html special character to text field and then later use javascript to get it back in its original format: like if I sent "&pi", it will shows "π" on the text input, and when I use javascript to get it back, I should get "&pi", but I only can get "π", not "&pi". Please help, my code is like the following:

<script type="text/javascript"> 
function qpush(a) {
    document.getElementById('input').value += a;
}

function show(a) {
    alert(document.getElementById('input').value);
}
</script>

<input type="text" id="input" />
<input type="button" onclick="qpush('&pi;');" value="&pi;" />
<input type="button" onclick="show()" value="go" />