The game of Tetris is played with four-connected blocks falling down the well having N rows and 20 columns. Each figure is marked with a unique English letter from 'A' to 'Z'.
Your program must, given the state of the well, determine the order in which the blocks fell down.
Input file format
The first line of input file contains integer N - number of rows. Following N
lines contain 20 characters each, with characters that are either a letter
from 'A' to 'Z' or the dot character (ASCII 46), representing an empty cell.
Output file format
Output file must contain a string of letters indicating the order in which
figures fell down. If there is more than one order, lexicographically
smallest one must be printed. Input data will guarantee that at least one
nonempty order exists.