js判断按钮a被点击加1分，按钮b被点击扣1分

html代码：
<div class="box">你现在的分数：<span id="mymark"></span></div>
<input type="button" id="plus" value="a" />
<input type="button" id="reduce" value="b" />

js代码：
var abtn = document.getElementById("plus");
var bbtn = document.getElementById("reduce");
var mymark = 0;
var mark = document.getElementById("mymark");
abtn.onclick = function(){
if (mark.innerHTML != "") {
mymark++;
}
mark.innerHTML = mymark;
}
bbtn.onclick = function(){
if (mark.innerHTML != "") {
mymark--;
}
mark.innerHTML = mymark;
}