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




if __name__ == '__main__':
    main()