Counter Script

This simple script counts up or down with a button click, now can it be
modified so the count won't go below zero?
           Thanks, Joe

<form>
<input type=text name=amount size=4 value=>
<input type=button value="up"
onClick="javascript:this.form.amount.value++;">
<input type=button value="down"
onClick="javascript:this.form.amount.value--;">
</form>

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