Cubes - 3
Vitek has at home 2 identical sets of cubes of English letters, but during the regular cleaning one of the cubes was lost. Help Witek to determine which of the cubes is missing in one of the sets.
The first line contains the number of found cubes n (1 ≤ n ≤
105) by Vitek, and the second line contains n characters depicted on each of the cubes.
Print the character on the lost cube, or the message "Ok", if Vitek was mistaken and no cube is lost.