koa 如何实现静态文件路由

const Koa = require('koa');
const app = new Koa();
const  serve = require("koa-static");

app.use(serve(__dirname+ "/static/html",{ extensions: ['html']}));

app.listen(3000);

const Koa = require('koa');
const app = new Koa();
const Router = require('koa-router');

let router = new Router()

router.get('/',async ctx => {
    ctx.body = '前台'
})
router.get('/admin',async ctx => {
    ctx.body = '后台'
})

server.use(router.routes())

app.listen(3000);