|hash|Array of [long\|string](/API_docs/types/long|string.html) | If there already is a full contact list on the client, a [hash](https://core.telegram.org/api/offsets#hash-generation) of a the list of contact IDs in ascending order may be passed in this parameter. If the contact set was not changed, [(contacts.contactsNotModified)](../constructors/contacts.contactsNotModified.html) will be returned. | Optional|