Vue js;Vue router is not constructor error in simple route

I am trying to implement simple routing with official route library.

Here is my app.js

 window.Vue = require('vue');
    window.VueRouter= require('vue-router');

    Vue.use(VueRouter);

const vt=Vue.component('example', require('./components/Example.vue'));

const router = new VueRouter({
      routes:[
        {
          path:'/test',
          component:vt
        }
    ]
})

const app = new Vue({
    el: '#app',
    router:router
});

But i got an error

app.js:11256 Uncaught TypeError: VueRouter is not a constructor

via Chebli Mohamed