Angularjs Controller and Expression Behaviour

I’m very new to angularjs and going through the tutorials. I’m held up at a point. In the below code why is {{result}} value not being displayed. What is the flow of code in angularjs:

<script src=""></script>

<div ng-app="myApp" ng-controller="myCtrl">
{{ result }}

var app = angular.module("myApp", []);
app.controller("myCtrl", function($scope) {
$scope.firstNum = 6;
$scope.lastNum = 7;


Here is the code

Source: AngularJS