This change in the app.js file works as desired:

/*angular.module('app', [].controller('MainController', function() {
    this.num1 = 0;
    this.num2 = 0;
}));*/

var app = angular.module('app', []);
app.controller('MainController', function() {
    this.num1 = 0;
    this.num2 = 0;  
});