Conflicting migrations

CommandError: Conflicting migrations detected; multiple leaf nodes in the migration graph: (0015_announcement_recipient_extensible_model, 0013_multiple_recipients_announcement in core). To fix them run 'python manage.py makemigrations --merge'

Merging core
  Branch 0015_announcement_recipient_extensible_model
    - Remove field created_at from activity
    - Remove field created_at from notification
    - Add field extended_data to activity
    - Add field extended_data to announcement
    - Add field extended_data to notification
    - Change Meta options on announcement
    - Remove field content_type from announcement
    - Remove field recipient_id from announcement
    - Alter field description on announcement
    - Alter field valid_until on announcement
    - Create model AnnouncementRecipient
    - Change Meta options on announcementrecipient
    - Add field extended_data to announcementrecipient
  Branch 0013_multiple_recipients_announcement
    - Change Meta options on announcement
    - Remove field content_type from announcement
    - Remove field recipient_id from announcement
    - Alter field description on announcement
    - Alter field valid_until on announcement
    - Create model AnnouncementRecipient
    - Change Meta options on announcementrecipient

Merging will only work if the operations printed above do not conflict
with each other (working on different fields or models)
Do you want to merge these migration branches? [y/N]