how to validate checkbox(mandatory) using angularjs?

This page listed stores with checkboxes .i need to put required field for checkbox if nothing is selected in that checkbox i need to put error message this field mandatory and nothing is selected submit button should disabled if user selected any one in that store list mandatory message should disappear and submit button should be enable.i have already done but when i checked 3 store then only message disappear and submit button enabled. Expectation:when user checked anyone listed checkbox message should disappear and submit button should enable.if nothing is checked in that checkbox message apper and submit button should be disbled.

function Test1Controller($scope) {
  
            var serverData = ["Store1", "Store2", "Store3"];
            $scope.items= [] ;
    
    for(var i=0;i<serverData.length;i++){
        var modal = {
        name:serverData[i],
        selected:false
    };        
        $scope.items.push(modal);        
    }
            
            $scope.check = function()
            {
                var checkedItems = [];
                for(var i=0;i<$scope.items.length;i++){
                    if($scope.items[i].selected){
                        checkedItems.push($scope.items[i].name);
                    }
                }
                console.log(checkedItems);
            }

    
}

<script src="http://ift.tt/1bdM3QL"></script>

<div ng-app>
<form name="projectForm">
  <div ng-controller="Test1Controller" >
<div ng-repeat="item in items">
<input type="checkbox" ng-model="item.selected" required="" /> {{item.name}}
</div>
<div class="form-error" ng-messages="projectForm.booktime.$error">
                                <div class="form-error" ng-message="required">* Mandatory</div>
                                        
                        </div>
<input type="button" name="submit" value="submit" ng-disabled="projectForm.$invalid" ng-click="check()"/>
</div>
</div>
</form>