H, W = map(int, input().split())
S = [input() for _ in range(H)]
T = [S[0][0]]
Q = [(0, 0)]

for _ in range(H + W - 2):
    Q2 = []
    h0 = -1
    c = "z"
    for h,w in Q:
        if w < W - 1 and h > h0:
            if S[h][w + 1] == c:
                Q2.append((h,w + 1))
            elif S[h][w + 1] < c:
                c = S[h][w + 1]
                Q2 = [(h,w + 1)]
            h0 = h
        if h < H - 1 and h + 1 > h0:
            if S[h + 1][w] == c:
                Q2.append((h + 1,w))
            elif S[h + 1][w] < c:
                c = S[h + 1][w]
                Q2 = [(h + 1,w)]
            h0 = h + 1
    #print(Q2)
    Q = Q2
    T.append(c)

print("".join(T))