Hi I’m new new in Angularjs library I’m begin to learn AngularJs Controller but my Controller is not work

My AngularJs Controller Code:

mainApp.controller('employeeController', function ($scope) {
        $scope.employee = {
            firstName: "Kamlakar",
            lastName: "Singh",
            fullName: function () {
                var studentObject;
                employeeObject = $scope.employee;
                return employeeObject.firstName + " " + employeeObject.lastName;
            }
        };
    });

My AngularJs Html Code:

<div ng-app=" " ng-controller="employeeController">
 
Enter first name: <input type="text" ng-model="employee.firstName"><br><br>
Enter last name: <input type="text" ng-model="employee.lastName"><br>
<br>
You are entering: {{employee.fullName()}}
</div>

Where is problem I can’t understand please solve my problem as soon as possible.

Thank you in Advance: