## https://yukicoder.me/problems/no/2064


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

    current = [(0, 0)]
    answer = [S[0][0]]
    for _ in range(H + W - 2):
        max_word = "wwww"
        max_list = []
        for ci, cj in current:

            for di, dj in ((1, 0), (0, 1)):
                next_ci = ci + di
                next_cj = cj + dj
                if 0 <= next_ci < H and 0 <= next_cj < W:
                    s = S[next_ci][next_cj]
                    if max_word == s:
                        max_list.append((next_ci, next_cj))
                    elif max_word > s:
                        max_word = s
                        max_list = [(next_ci, next_cj)]

        answer.append(max_word)
        current = list(set(max_list))
    
    print("".join(answer))






if __name__ == "__main__":
    main()