                        课程
                    
                        /前端开发
                        
                            /JavaScript
                        
                        /JavaScript进阶篇

求和结果为什么是nan?

<!DOCTYPE  HTML>
<html >
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>系好安全带,准备启航</title>

<script type="text/javascript">

  var mydate=new Date();
  var weekday=["星期日","星期一","星期二","星期三","星期四","星期五","星期六",];
  document.write(mydate.getFullYear()+"年"+parseFloat(mydate.getMonth()+1)+"月"+mydate.getDate()+"日 "+weekday[mydate.getDay()]+"<br>");
 
  
  var scoreStr = "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 score1=new Array;
  var score2=new Array;
  var sum;
  score1=scoreStr.split(";");
  for(var i=0;i<score1.length;i++){
      document.write(score1[i]+"<br>")
    score2=score1[i].split("");
    var a=score2[3]+score2[4];
    document.write(a+"<br>")
    sum=sum+parseInt(a);    
  }
 
  sum=Math.round(sum/10);
  document.write(sum);
 
</script>
</head>
<body>
</body>
</html>