求帮忙，哪里错了喃，就是运行不出来
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var num1=document.getElementById("txt1").value;
var num1=document.getElementById("txt2").value;
var str=document.getElementById("select").value;
var str1="";
switch (str){
case "+":
str1=parseFloat(num1) + parseFloat(num2);
break;
case "-":
str1=parseFloat(num1) - parseFloat(num2);
break;
case "*":
str1=parseFloat(num1) * parseFloat(num2);
break;
case "/":
str1=parseFloat(num1) / parseFloat(num2);
break;
}
document.getElementById("fruit").value = str1;
}
</script>
</head>
<body>
<input type="text" id="txt1" />
<select id="select">
<option value="+">+</option>
<option value="-">-</option>
<option value="*">*</option>
<option value="/">/</option>
</select>
<input type="text" id="txt2" />
<input type="button" value=" = " onclick="count()" /> <!--通过 = 按钮来调用创建的函数，得到结果-->
<input type="text" id="fruit" />
</body>
</html>
