def main():
    H, W = map(int, input().split())
    letters = [list(input()) for _ in range(H)]

    strings = ""
    current_h, current_w = 0, 0
    while current_h < (H-1) or current_w < (W-1):
        strings += letters[current_h][current_w]

        if current_h == H-1 or current_w == W-1:
            current_w = min(current_w+1, W-1)
            current_h = min(current_h+1, H-1)

        elif letters[current_h+1][current_w] < letters[current_h][current_w+1]:
            current_h += 1
        else:
            current_w += 1

    strings += letters[H-1][W-1]
    print(strings)


if __name__ == "__main__":
    main()