Statement

Professor BP loves palindromes. He wrote a sequence V of N positive integers on the blackboard and was delighted to see that it contained long palindromic substrings. However, during the break, one of his students erased some of the numbers from the sequence. Now, Professor BP wants to restore the sequence by replacing all the erased numbers with the same number x of his choice. Your task is to determine the length of the longest palindromic contiguous substring that can be obtained after choosing an optimal value for x.