How to use ng-required in ng-messages in AngularJS

How can we get required validation conditionally in ng-messages

Previously i had used ng-required for input control conditionally

$scope.IsRequired=true;

<input type="text" name="empname" ng-model="EmpName" ng-required="IsRequired" />

the above is without using ng-messages,

i need to use ng-messages, then how can i use ng-required conditionally.

please help me... thanks in advance