Map<Id,Event> ObjYourMAp  = new Map<Id,Event>();
// add some values in Map
Map <Id,Event> MapLeadEvent = new Map <Id,Event>();
for(Event objEvt : ObjYourMAp.Values())
{
    //-----------------getObjectName
    String sObjName = (objEvt.WhoId).getSObjectType().getDescribe().getName();
    if(sObjName == 'Lead')
    {
        if(MapLeadEvent != null && MapLeadEvent.containsKey(objEvt.Id) )
        {
            if(MapLeadEvent.get(objEvt.Id).CreatedDate < objEvt.CreatedDate)
                MapLeadEvent.put(objEvt.WhoId,objEvt);
        }
        else
        {
            MapLeadEvent.put(objEvt.WhoId,objEvt);
        }
    }
}