mod = 998244353


def main():
    import sys
    input = sys.stdin.readline

    H, W = map(int, input().split())
    G = []
    for _ in range(H):
        G.append(input().rstrip('\n'))

    ans = [G[0][0]]
    ok = [[0] * W for _ in range(H)]
    ok[0][0] = 1
    for hw in range(H + W - 2):
        mi = 27
        for h in range(H):
            w = hw - h
            if 0 <= w < W:
                if not ok[h][w]:
                    continue
                if h+1 < H:
                    mi = min(mi, ord(G[h+1][w]) - 97)
                if w + 1 < W:
                    mi = min(mi, ord(G[h][w+1]) - 97)
        for h in range(H):
            w = hw - h
            if 0 <= w < W:
                if not ok[h][w]:
                    continue
                if h + 1 < H:
                    if ord(G[h+1][w]) - 97 == mi:
                        ok[h+1][w] = 1
                if w + 1 < W:
                    if ord(G[h][w+1]) - 97 == mi:
                        ok[h][w+1] = 1
        ans.append(chr(mi + 97))
    print("".join(ans))


if __name__ == '__main__':
    main()