Akka streams: how to use backoff strategy only in case of failure?

I have a rather complex Source source, which I've wrapped into Backoff to be able to restart the stream in case of failure:

val backoffSource = RestartSource.withBackoff(settings){() => source}

I link this source to the sink, which has a materialized value Future[Done]:

val future = backoffSource.toMat(sink)(Keep.right).run()

After that in case of success of this graph I want do some actions when the graph is finished. So, I want to do somethink like:

future.onComplete{
  case Failure(ex) => println(ex)
  case Success(_) => println("Success")
}

But as per documentation RestartSource is

starting an operator again when it fails or completes, each time with a growing time delay between restarts

So my graph will keep restarting until maxRestarts is reached.

How can I implement the next behaviour:

1. Backoff strategy will be applied only in case of failure
2. In case of graph's success It will call my onComplete method with Success(_)
3. In case of maxRestarts failures the onComplete with Failure(_) will be called ?