You are given an $n \times n$ grid whose each square contains a letter.
You should move from the upper-left square to the lower-right square. You can only move right or down.
What is the lexicographically minimal string you can construct?
The first line has an integer $n$ : the size of the grid.
After this, there are $n$ lines that describe the grid. Each line has $n$ letters between A and Z.
Print the lexicographically minimal string.