Golang convert int or string to int

gistfile1.txt

package main

import (
	"encoding/json"
	"fmt"
	"log"
)

type Object struct {
	Num json.Number
}

func main() {
	o := &Object{}
	err := json.Unmarshal([]byte(`{"Num": "9223372036854775807"}`), o)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(o.Num.Int64())

	err = json.Unmarshal([]byte(`{"Num": 9223372036854775807}`), o)
	if err != nil {
		log.Println(err)
	}
	fmt.Println(int(o.Num.Int64()))
}