大佬们 为什么输出是NaN,帮帮忙
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>综合测试</title>
</head>
<body>
<script>
var mydate=new Date();
var times="";
times=mydate.getFullYear()+"年";
times+=mydate.getMonth()+1+"月";//添加加号
times+=mydate.getDate()+"日";
var day=mydate.getDay();
var week="";
switch(day){
case 0:
week="星期日";
break;
case 1:
week="星期一";
break;
case 2:
week="星期二";
break;
case 3:
week="星期三";
break;
case 4:
week="星期四";
break;
case 5:
week="星期五";
break;
case 6:
week="星期六";
break;
default://记得加！！！！
}
document.write(times+" "+week+"<br>");
var shuju="小明:87; 小花:81; 小红:97; 小天:76;小张:74;小小:94;小西:90;小伍:76;小迪:64;小曼:76";
var shuju1=shuju.split(";");
var sum=0;
var av=0;
for(var i=0;i<shuju1.length;i++){
var a=shuju1[i].substring[3,4];
sum+=a;
}
av=sum/shuju1.length;
document.write(sum+av);
</script>
</body>
</html>
