## 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_words = {}
        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]
                    max_words[(next_ci, next_cj)] = s

        max_words_array = [(pos, w) for pos, w in max_words.items()]
        max_words_array.sort(key=lambda x : x[1])

        s0 = max_words_array[0][1]
        answer.append(s0)
        next_current = []
        for pos, s in max_words_array:
            if s == s0:
                next_current.append(pos)
        current = next_current
    
    print("".join(answer))






if __name__ == "__main__":
    main()