#include <cstdio>

#include <cstdint>
#include <queue>
#include <string>
#include <typeinfo>
#include <cassert>
struct Coord {
  int r, c;
  Coord(int r_, int c_) : r(r_), c(c_) {}
};

Coord operator + (Coord x, Coord y) {
  return Coord(x.r + y.r, x.c + y.c);
}

template<typename type, uint32_t size>
struct Grid {
  type v[size][size];
  int h, w;
  type& operator[] (Coord x) {
    return v[x.r][x.c];
  }
};

template<typename a, uint32_t size>
void load(Grid<a, size>& grid, std::string format) {
  for(int i = 0; i < (int)format.size(); ++i) {
    if( format[i] == 'w' ) scanf("%d", &grid.w);
    if( format[i] == 'h' ) scanf("%d", &grid.h);
  }
  for(int r = 0; r < grid.h; ++r) {
    for(int c = 0; c < grid.w; ++c) {
      if( typeid(a) == typeid(int) ) scanf("%d", &grid[Coord(r, c)]);
      else assert(0);
    }
    while( getchar() != '\n' ) ;
  }    
}

const Coord d4[] = {Coord(1, 0), Coord(0, 1), Coord(-1, 0), Coord(0, -1)};
struct inRange_ {
  int h, w;
  inRange_(int h_, int w_) : h(h_), w(w_) {}
  template<typename a, uint32_t size> inRange_(Grid<a, size> grid) : h(grid.h), w(grid.w) {}
  bool operator()(Coord x) {
    return 0 <= x.r and x.r < h and 0 <= x.c and x.c < w;
  }
};

template<typename a, uint32_t size>
int fillGridCC(Grid<a, size>& grid, Coord x, a to) {
  inRange_ inRange(grid);
  std::queue<Coord> q;
  a from = grid[x];
  q.push(x);
  int count = 0;
  grid[x] = to;
  while( not q.empty() ) {
    Coord y = q.front(); q.pop();
    for(int k = 0; k < 4; ++k) {
      Coord z = y + d4[k];
      if( not inRange(z) ) continue;
      if( grid[z] != from ) continue;
      grid[z] = to;
      count += 1;
      q.push(z);
    }
  }
  return count + 1;
}

template<typename a, uint32_t size>
bool fillGridCCandCheckLoop(Grid<a, size>& grid, Coord x, a to) {
  inRange_ inRange(grid);
  std::queue<Coord> q;
  a from = grid[x];
  q.push(x);
  grid[x] = to;
  bool res = false;
  while( not q.empty() ) {
    Coord y = q.front(); q.pop();
    int count = 0;
    for(int k = 0; k < 4; ++k) {
      Coord z = y + d4[k];
      if( not inRange(z) ) continue;
      if( grid[z] == to ) count += 1;
      if( grid[z] != from ) continue;
      grid[z] = to;
      q.push(z);
    }
    if( count >= 2 ) res = true;
  }
  return res;
}

Grid<int, 128> grid;

int main() {
  load(grid, "wh");
  int w = grid.w, h = grid.h;
  // scanf("%d %d", &w, &h);
  // grid.w = w; grid.h = h;
  // for(int r = 0; r < h; ++r) {
  //   for(int c = 0; c < w; ++c) {
  //     scanf("%d", &grid[Coord(r, c)]);
  //   }
  // }

  bool res = 0;
  for(int r = 0; r < h; ++r) {
    for(int c = 0; c < w; ++c) {
      Coord p(r, c);
      if( grid[p] > 0 ) {
        res = res or fillGridCCandCheckLoop(grid, p, -grid[p]);
      }
    }
  }

  puts(res ? "possible" : "impossible");
  
  return 0;
}