Enable button when i have selected two values - angularjs

Question

I have two select i want to enable my button when i have selected values in my two select, i don't know how i can do this ?
JSFIDDLE
My HTML:
<div ng-app="myapp">
<fieldset ng-controller="FirstCtrl">
<select
ng-options="people.first for people in people_1"
ng-model="selectedPerson1"></select>
<select
ng-options="people.first for people in people_2"
ng-model="selectedPerson2"></select>
</fieldset>
<br/><button class="btn btn-primary" ng-click="platform=true">Comparer</button>
</div>

Answer 1

You can do this using the ng-disabled directive. I have amended your jsfiddle here: http://jsfiddle.net/p0t4cw84/3/. I moved the button into the scope of the controller FirstCtrl and added a method on the scope that will return true or false if values are selected.
$scope.enableCompare = function () {
return !($scope.selectedPerson1 && $scope.selectedPerson2);
};
I hope this helps.

Answer 2

Here you go.There are lot of issue in you fiddle.I fixed it.
http://jsfiddle.net/purpz7u3/
<div ng-app="myapp">
<div ng-controller="FirstCtrl">
<fieldset >
<select
ng-options="people.first for people in people_1"
ng-model="selectedPerson1" ng-change="check()"></select>
<select
ng-options="people.first for people in people_2"
ng-model="selectedPerson2" ng-change="check()"></select>
</fieldset>
<br/><button class="btn btn-primary" ng-disabled="checked">Comparer</button>
</div>
</div>
var myapp = angular.module('myapp', []);
myapp.controller('FirstCtrl', function ($scope) {
$scope.checked = true;
$scope.people_1 = [
{ id: 1, first: 'John' },
{ id: 2, first: 'Rocky' }
];
$scope.people_2 = [
{ id: 1, first: 'Rambo' },
{ id: 2, first: 'Balboa'}
];
$scope.check = function(){
if($scope.selectedPerson1 && $scope.selectedPerson2){
$scope.checked=false;
}
}
});