local h, w = io.read("*n", "*n", "*l")
local t = {}
for i = 1, h do
  local s = io.read()
  t[i] = {}
  for j = 1, w do
    t[i][j] = s:sub(j, j)
  end
end
local ret = {}
local last = t[1][1]
local pos = {}
pos[1] = true
ret[1] = last
for len = 2, h + w - 1 do
  local nxt = "~"
  for i = 1, h do
    if pos[i] then
      local j = len - i
      if j < w and t[i][j + 1] < nxt then
        nxt = t[i][j + 1]
      end
      if i < h and t[i + 1][j] < nxt then
        nxt = t[i + 1][j]
      end
    end
  end
  local tmp = {}
  for i = h, 1, -1 do
    if pos[i] then
      local j = len - i
      if j < w and t[i][j + 1] == nxt then
        tmp[i] = true
      end
      if i < h and t[i + 1][j] == nxt then
        tmp[i + 1] = true
      end
    end
  end
  pos = {}
  for k, v in pairs(tmp) do
    pos[k] = true
  end
  table.insert(ret, nxt)
  last = nxt
end
print(table.concat(ret))