Problem

<html lang="ru">
<head>
<title>Problem</title>
</head>
<body>
<script>
var speed = 2000;
</script>
<div id="mainBorder" >
<input type=button onclick="deploy()" value="Нажать">
 </div>
 <input type=form id= "forSpeed" >
   <script>
   function deploy(){
           var speed = speed - 100 ;
                    };
</script>
<script>
document.getElementById('forSpeed').value = speed;
</script>
</body>

</html>