angular js "Cannot GET"

This is my web.js For some reason, I can't access http://localhost:8000/app/

var port = Number(process.env.PORT || 8000);

var express = require('express');
var app = express();

app.use(express.static(__dirname + '/app'));
var server = app.listen(port, function() {
    console.log('Listening on port %d', server.address().port);
});