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

ans = []
s = set()
s.add((0,0))

ans.append(S[0][0])
for _ in range(H + W - 2):
    nx = set()
    c = "z"
    for x,y in s:
        if x + 1 < H:
            u = S[x + 1][y]
            if u < c:
                c = u
        if y + 1 < W:
            u = S[x][y + 1]
            if u < c:
                c = u
    ans.append(c)
    for x,y in s:
        if x + 1 < H and S[x + 1][y] == c:
            nx.add((x + 1,y))
        if y + 1 < W and S[x][y + 1] == c:
            nx.add((x,y + 1))
    s = nx
print("".join(ans))