A1331.[COCI-2007_2008-contest2]#5 PRVA

Little Ivica solves crossword puzzles every day. In case you haven't seen one, a crossword puzzle starts on a grid of R×C squares, each of which is either empty or blocked. The player's task is to write words in consecutive empty squares vertically (top down) or horizontally (left to right).
Ivica's sister has a strange habit of looking at crosswords Ivica has finished solving, and finding the lexicographically smallest word in it. She only considers words at least 2 characters long.
Write a program that, given a crossword puzzle, finds that word.

The first line contains two integers R and C (2 ≤ R, C ≤ 20), the number of rows and columns in the crossword.
Each of the following R lines contains a string of C characters. Each of those characters is either a lowercase letter of the English alphabet, or the character '#' representing a blocked square.
The input will be such that a solution will always exist.

Output the lexicographically smallest word in the crossword.