Three friends like to play the following game. The first friend chooses a string S. Then the second friend constructs a new string T that consists of two copies of the string S. finally, the third friend inserts one letter at the beginning, the end or somewhere inside the string T, thereby creating a string U. You are given the string U and your task is to reconstruct the original string S.
The first line of the input contains N（2 ≤ N ≤ 2000001）, the length of the final string U. The string U itself is given on the second line. It consists of N uppercase English letters (A, B, C, . . . , Z).
Your program should print the original string S. However, there are two exceptions: 1.If the final string U could not have been created using the above procedure, you should print NOT POSSIBLE. 2.If the original string S is not unique, you should print NOT UNIQUE.