def main():
    h, w = map(int, input().split())
    A = [input() for _ in range(h)]
    B = [[0] * w for _ in range(h)]
    d = {A[0][0]: [(0, 0)]}
    for _ in range(h + w - 1):
        s = min(d)
        q = [(i, j) for i, j in d[s]]
        d.clear()
        while q:
            i, j = q.pop()
            B[i][j] = 1
            if i < h - 1:
                a = A[i + 1][j]
                if a in d:
                    d[a].append((i + 1, j))
                else:
                    d[a] = [(i + 1, j)]
            if j < w - 1:
                a = A[i][j + 1]
                if a in d:
                    d[a].append((i, j + 1))
                else:
                    d[a] = [(i, j + 1)]
    ans = []
    i, j = h - 1, w - 1
    for _ in range(h + w - 1):
        ans.append(A[i][j])
        if i > 0 and B[i - 1][j]:
            i -= 1
        else:
            j -= 1
    print(''.join(reversed(ans)))




if __name__ == '__main__':
    main()