// URL: https://yukicoder.me/problems/no/13

import std.algorithm, std.array, std.container, std.math, std.range, std.typecons, std.string;

version(unittest) {} else
void main()
{
  int W, H; io.getV(W, H);
  int[][] M; io.getM(H, W, M);

  auto g = grid(M), v = g.grid!bool;
  alias Pos = g.Pos;

  auto hasCycle(Pos p)
  {
    struct Pos2 { Pos curr, prev; }
    auto q = DList!Pos2(Pos2(p, p)); v[p] = true;

    while (!q.empty) {
      auto e = q.front; q.removeFront();
      foreach (d; e.curr.around4.filter!(d => d != e.prev && g[d] == g[p])) {
	if (v[d]) return true;
	q.insertBack(Pos2(d, e.curr)); v[d] = true;
      }
    }

    return false;
  }

  foreach (p; g.walk)
    if (!v[p] && hasCycle(p)) io.put!"{exit: true}"("possible");

  io.put("impossible");
}

template Grid(alias h, alias w)
{
  struct Pos
  {
    int r, c;
    this(int r, int c) { this.r = r; this.c = c; }
    bool inGrid() { return 0 <= r && r < h && 0 <= c && c < w; }
    int p2i() { return cast(int)w*r+c; }

    pure Pos opBinary(string op)(Pos q) if (op=="+"||op=="-")
    { return mixin("Pos(r"~op~"q.r, c"~op~"q.c)"); }
    Pos opOpAssign(string op)(Pos q) if (op=="+"||op=="-")
    { mixin("r"~op~"=q.r; c"~op~"=q.c;"); return this; }
    pure Pos opBinary(string op, U)(U a) if (op=="*"||op=="/")
    { return mixin("Pos(r"~op~"a, c"~op~"a)"); }
    Pos opOpAssign(string op, U)(U a) if (op=="*"||op=="/")
    { mixin("r"~op~"=a; c"~op~"=a;"); return this; }
    pure int opBinary(string op: "*")(Pos q)
    { return r*q.r+c*q.c; }

    pure auto around4()
    {
      return [Pos(r-1, c), Pos(r, c+1), Pos(r+1, c), Pos(r, c-1)]
        .filter!(p => p.inGrid);
    }
    pure auto around8()
    {
      return [Pos(r-1, c), Pos(r-1, c+1), Pos(r, c+1), Pos(r+1, c+1),
              Pos(r+1, c), Pos(r+1, c-1), Pos(r, c-1), Pos(r-1, c-1)]
        .filter!(p => p.inGrid);
    }
  }

  struct Data(T)
  {
    alias Pos = Grid!(h, w).Pos;

    T[][] data;

    this(T[][] data) { h = data.length; w = data[0].length; this.data = data; }
    pure Data!T dup() { return Data!T(data.map!"a.dup".array); }

    pure Pos pos(int r, int c) { return Pos(r, c); }
    pure Data!U grid(U)() { return Data!U(new U[][](h, w)); }

    pure T opIndex(size_t r, size_t c) { return data[r][c]; }
    pure T opIndex(Pos p) { return data[p.r][p.c]; }
    Data!T opIndexAssign(T v, size_t r, size_t c) { data[r][c] = v; return this; }
    Data!T opIndexAssign(T v, Pos p) { data[p.r][p.c] = v; return this; }
    Data!T opIndexOpAssign(string op)(T v, size_t r, size_t c)
    { mixin("data[r][c]"~op~"=v;"); return this; }
    Data!T opIndexOpAssign(string op)(T v, Pos p)
    { mixin("data[p.r][p.c]"~op~"=v;"); return this; }
    Data!T opIndexUnary(string op)(size_t r, size_t c) if (op=="++"||op=="--")
      { mixin(op~"data[r][c];"); return this; }
    Data!T opIndexUnary(string op)(Pos p) if (op=="++"||op=="--")
      { mixin(op~"data[p.r][p.c];"); return this; }

    auto walk()
    { return WalkRange(this); }

    private struct WalkRange
    {
      int r, c;
      this(Data!T g) { r = 0; c = 0; }
      @property pure Pos front() { return Pos(r, c); }
      void popFront() { if (++c >= w) { c = 0; ++r; } }
      pure bool empty() { return r >= h; }
    }
  }
}

auto grid(T)(size_t h, size_t w)
{ return Grid!(h, w).Data!T(new T[][](h, w)); }
auto grid(T)(T[][] data)
{ auto h = data.length, w = data[0].length; return Grid!(h, w).Data!T(data); }

pure int distManhattan(T)(T p1, T p2) { return abs(p1.r-p2.r) + abs(p1.c-p2.c); }

auto io = IO!()();
import std.stdio;
struct IO(alias IN = stdin, alias OUT = stdout)
{
  import std.conv, std.format, std.meta, std.traits, core.stdc.stdlib;

  auto getV(T...)(ref T v) { foreach (ref w; v) get(w); }
  auto getA(T)(size_t n, ref T v) if (hasAssignableElements!T)
  { v = new T(n); foreach (ref w; v) get(w); }
  auto getC(T...)(size_t n, ref T v)
  if (allSatisfy!(hasAssignableElements, T))
  {
    foreach (ref w; v) w = new typeof(w)(n);
    foreach (i; 0..n) foreach (ref w; v) get(w[i]);
  }
  auto getM(T)(size_t r, size_t c, ref T v)
  if (hasAssignableElements!T && hasAssignableElements!(ElementType!T))
  { v = new T(r); foreach (ref w; v) getA(c, w); }
  template getS(E...)
  {
    auto getS(T)(size_t n, ref T v)
    { v = new T(n); foreach (ref w; v) foreach (e; E) mixin("get(w."~e~");"); }
  }

  const struct PutConf
  {
    bool newline = true, flush, exit;
    string floatFormat = "%.10f", delimiter = " ";
  }

  auto put(alias conf = "{}", T...)(T v)
  { mixin("const PutConf c = "~conf~"; putMain!c(v);"); }
  auto putB(alias conf = "{}", S, T)(bool c, S t, T f)
  { if (c) put!conf(t); else put!conf(f); }
  auto putRaw(T...)(T v) { OUT.write(v); OUT.writeln; }

  private
  {
    dchar[] buf;
    auto sp = (new dchar[](0)).splitter;
    void nextLine() { IN.readln(buf); sp = buf.splitter; }
    auto get(T)(ref T v) { if (sp.empty) nextLine(); v = sp.front.to!T; sp.popFront(); }

    auto putMain(PutConf c, T...)(T v)
    {
      foreach (i, w; v) {
        putOne!c(w);
        if (i < v.length-1) OUT.write(c.delimiter);
      }
      static if (c.newline) OUT.writeln;
      static if (c.flush) OUT.flush();
      static if (c.exit) exit(0);
    }
    auto putOne(PutConf c, T)(T v)
    {
      static if (isInputRange!T && !isSomeString!T) putRange!c(v);
      else if (isFloatingPoint!T) OUT.write(format(c.floatFormat, v));
      else OUT.write(v);
    }
    auto putRange(PutConf c, T)(T v)
    {
      auto w = v;
      while (!w.empty) {
        putOne!c(w.front); w.popFront();
        if (!w.empty) OUT.write(c.delimiter);
      }
    }
  }
}