// http://yukicoder.me/problems/37
open System
open System.Collections.Generic

let dpfn fmt = Printf.kprintf Diagnostics.Debug.WriteLine fmt

let dir = [1,0; 0,-1; -1,0; 0,1]

[<EntryPoint>]
let main argv = 
    let W, H = Console.ReadLine().Trim().Split([|' '|]) |> Array.map int |> fun a -> a.[0], a.[1]
    let M = Array.init H (fun _ -> Console.ReadLine().Trim().Split([|' '|]) |> Array.map int)
    let memo = Array2D.init W H (fun _ _ -> false)
    let rec check_loop c hist queue =
        if Set.count queue = 0 then false else
        let depth, x, y = Set.maxElement queue
        let queue' = Set.remove (depth, x, y) queue
        memo.[x,y] <- true
        if Map.tryFind (x,y) hist <> None then true else
        let hist' = Map.add (x,y) depth hist
        let queue' =
            dir
            |> List.map (fun (dx,dy) -> x+dx,y+dy)
            |> List.filter (fun (x,y) -> x>=0 && y>=0 && x<W && y<H)
            |> List.filter (fun (x,y) -> M.[y].[x] = c &&
                                         (memo.[x,y] = false || 
                                          Map.tryFind (x,y) hist |> function | Some d -> depth+1-d>=4 | None -> false))
            |> List.fold (fun q (x,y) -> Set.add (depth+1,x,y) q) queue'
        check_loop c hist' queue'
    let rec loop x y =
        if y = H then false else
        if x = W then loop 0 (y+1) else
        if memo.[x,y] then loop (x+1) y else
        let ans = check_loop M.[y].[x] Map.empty (Set.singleton (0,x,y))
        if ans then ans else loop (x+1) y
    if loop 0 0 then "possible" else "impossible"
    |> printfn "%s"
    0 // 整数の終了コードを返します