案例中对字数的监听如果按退格键也会将数字算入其中，该如何避免
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>vue.js小案例</title>
</head>
<body>
<div id="box">
<input type="text" v-model="num1">
<input type="text" v-model="num2">
<h3 v-text="sum"></h3>
</div>
</body>
<script src="js/Vue.js"></script>
<script>
new Vue({
el: "#box",
data: {
num1: '',
num2: '',
sum: 0
},
watch: {
num1: function () {
this.sum = this.num2.length?parseInt(this.num1) + parseInt(this.num2):parseInt(this.num1);
},
num2: function () {
this.sum = this.num1.length?parseInt(this.num1) + parseInt(this.num2):parseInt(this.num1)
}
}
})
</script>