H, W = map(int, input().split())
grid = [list(input().strip()) for _ in range(H)]

current = [(0, 0)]
result = [grid[0][0]]

for _ in range(H + W - 2):
    next_cells = set()
    for (i, j) in current:
        if i + 1 < H:
            next_cells.add((i + 1, j))
        if j + 1 < W:
            next_cells.add((i, j + 1))
    
    min_char = min(grid[i][j] for (i, j) in next_cells)
    
    filtered = [(i, j) for (i, j) in next_cells if grid[i][j] == min_char]
    current = filtered
    result.append(min_char)

print(''.join(result))