<script> 
function doresult(){ 
if(document.form1.num.value==''){ 
alert('请输入一个数字'); 
return false; 
}else 
{ 
var re=/[^0-9]+/g; 
if(re.test(document.form1.num.value)){ 
alert('请输入一个数字'); 
return false; 
} 
} 
var thenum=parseInt(document.form1.num.value); 
var theresult=(thenum-1)*4+1024*thenum; 
document.getElementById('theshow').innerText=theresult.toString(); 
} 
</script> 
<form name=form1> 
<input name=num onkeyup="this.value=this.value.replace(/[^\\d]*/g,'')" onbeforepaste="clipboardData.setData('text',clipboardData.getData('text').replace(/[^\\d]*/g,''))"> 
<span id=theshow style="border:1px solid green;margin:0 5px"></span> 
<button onclick="javascript:doresult()"><font color=red>计算</font></button> 
</form>