from collections import deque

h, w = map(int, input().split())
s = [input() for _ in range(h)]

dx = [1, 0]
dy = [0, 1]

cands = set()
cands.add((0, 0))
ans = [s[0][0]]
for i in range(h + w - 2):
  c = None
  nxt_cands = set()
  for cx, cy in cands:
    for j in range(2):
      nx = cx + dx[j]
      ny = cy + dy[j]
      if nx >= h or ny >= w:
        continue
      if c is None or c > s[nx][ny]:
        c = s[nx][ny]
        nxt_cands = set()
        nxt_cands.add((nx, ny))
      elif c == s[nx][ny]:
        nxt_cands.add((nx, ny))

  cands = nxt_cands
  ans.append(c)

print(*ans, sep='')