H, W = list(map(int, input().split()))
S = [input() for _ in range(H)]
T = ''
def pick_right(h, w):
  if w == W - 1:
    return None
  return S[h][w + 1]

def pick_below(h, w):
  if h == H - 1:
    return None
  return S[h + 1][w]

def move(h, w):
  right = pick_right(h, w)
  below = pick_below(h, w)
  if right is None and below is None:
    return None
  if right is None:
    return (h + 1, w)
  if below is None:
    return (h, w + 1)
  if right > below:
    return (h + 1, w)
  if right < below:
    return (h , w + 1)

flag = True
h, w = 0, 0
while(flag):
  T += S[h][w]
  p = move(h, w)
  if p is None:
    flag = False
    break
  h, w = p[0], p[1]
print(T)