I am new for golang. I met a problem when I use multiplication in html/template. Some code like below.

template code:

 {{range $i,$e:=.Items}}
      <tr>
           <td>{{add $i (mul .ID .Number)}}</td>
           <td>{{.Name}}</td>
      </tr>
  {{end}}

.go code

type Item struct{
        ID int
        Name string
    }
func init() {
    itemtpl,_:=template.New("item.gtpl").
        Funcs(template.FuncMap{"mul": Mul, "add": Add}).
        ParseFiles("./templates/item.gtpl")
}

func itemHandle(w http.ResponseWriter, req *http.Request) {

    items:=[]Item{Item{1,"name1"},Item{2,"name2"}}
    data := struct {
            Items []Item
            Number int
            Number2 int
        }{
            Items:    items,
            Number:   5,
            Number2:  2,
        }
        itemtpl.Execute(w, data)
}
func Mul(param1 int, param2 int) int {
    return param1 * param2
}
func Add(param1 int, param2 int) int {
    return param1 + param2
}

It will output nothing when I use the code above. But It will output 10 when I use the code outside of array below.

<html>
<body>
    {{mul .Number .Number2}}
</html>
</body>

I google a lot. I cannot find the usable like mine. I want to use multiplication in array inside of html/template. Can someone tell me what is wrong with my code?