[Deff]

<input type="text" name="vtb2" id="vtb2" size="4" maxlength="3" value="640" oninput="fn(this)">
 
<input type="text" name="vtb3" id="vtb3" size="4" maxlength="3" value="540" readonly="true">


 
<script>  
function fn(a){
	input=document.getElementById('vtb3');
	var b = parseInt(a.value);
 	    if(!isNaN(b)){
	      input.value=parseInt(b)-100;
	    }
}


</script>