local w, h = io.read("*n", "*n", "*l")
local str = ""
local col = {}
local done = {}
for i = 1, h do
  str = io.read()
  local idx = (i - 1) * w
  for v in str:gmatch("%d+") do
    idx = idx + 1
    col[idx] = tonumber(v)
    done[idx] = false
  end
end

local function search(startpos)
  local tasks = {startpos}
  local tasknum, donenum = 1, 0
  while donenum < tasknum do
    donenum = donenum + 1
    local idx = tasks[donenum]
    done[idx] = true
    local known = 0
    if w ~= 1 then
      if idx % w ~= 0 and col[idx + 1] == col[idx] then
        if done[idx + 1] then
          known = known + 1
        else
          table.insert(tasks, idx + 1)
          tasknum = tasknum + 1
        end
      end
      if idx % w ~= 1 and col[idx - 1] == col[idx] then
        if done[idx - 1] then
          known = known + 1
        else
          table.insert(tasks, idx - 1)
          tasknum =tasknum + 1
        end
      end
    end
    if h ~= 1 then
      if idx <= (h - 1) * w and col[idx + w] == col[idx] then
        if done[idx + w] then
          known = known + 1
        else
          table.insert(tasks, idx + w)
          tasknum = tasknum + 1
        end
      end
      if w < idx and col[idx - w] == col[idx] then
        if done[idx - w] then
          known = known + 1
        else
          table.insert(tasks, idx - w)
          tasknum =tasknum + 1
        end
      end
    end
    if 2 <= known then
      return true
    end
  end
  return false
end

local curpos = 1
local found = false
while curpos <= h * w do
  found = search(curpos)
  if found then break end
  curpos = curpos + 1
  while curpos <= h * w and done[curpos] do
    curpos = curpos + 1
  end
end
print(found and "possible" or "impossible")