After reading the asked questions about this topic I understand this: 1. The resulting chain mustn't be the longest 2. I assume that the chaining is at the beginning or at the end. I tested my code with the following input: CaaT Okinawa Washington Auckland Kalamazoo zoooZ zooe Norfolk The output was: Washington Norfolk Kalamazoo Okinawa Auckland Maybe I am missing something about the conditions of validation. Can I get a hint?