const INF: usize = 1usize << 60;
const DX: [isize; 4] = [-1,1,0,0];
const DY: [isize; 4] = [0,0,-1,1];

fn dfs(x: usize, y: usize, target: usize, grid: &Vec<Vec<usize>>, dists: &mut Vec<Vec<usize>>) -> bool {
    let h = grid.len();
    let w = grid[0].len();
    for direct in 0..4 {
        let nx = x as isize + DX[direct];
        let ny = y as isize + DY[direct];
        if nx >= 0 && ny >= 0 && nx < h as isize && ny < w as isize {
            let nx = nx as usize;
            let ny = ny as usize;
            if grid[nx][ny] != target { continue; }
            let nval = dists[x][y] + 1;
            if dists[nx][ny] < INF && (nval as isize - dists[nx][ny] as isize).abs() >= 3 {
                return true;
            }
            if dists[nx][ny] < INF { continue; }
            dists[nx][ny] = nval;
            if dfs(nx, ny, target, grid, dists) {
                return true;
            }
        }
    }
    false
}

fn main() {
    let mut hw = String::new();
    std::io::stdin().read_line(&mut hw).ok();
    let hw: Vec<usize> = hw.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let h = hw[1];
    let w = hw[0];
    let grid = (0..h).map(|_| {
            let mut temp = String::new();
            std::io::stdin().read_line(&mut temp).ok();
            let temp: Vec<usize> = temp.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
            temp
        })
        .collect::<Vec<Vec<usize>>>();

    let mut dists = vec![vec![INF; w]; h];
    for i in 0..h {
        for j in 0..w {
            if dists[i][j] < INF { continue; }
            dists[i][j] = 0;
            let target = grid[i][j];
            if dfs(i, j, target, &grid, &mut dists) {
                println!("possible");
                return;
            }
        }
    }
    println!("impossible");
}