#![allow(unused_imports)]

use std::io::{self, BufRead};
use std::str::FromStr;
use std::collections::*;
use std::cmp::*;

#[allow(dead_code)]
struct Parser<'a> {
    tokens: &'a mut Iterator<Item = String>,
}
#[allow(dead_code)]
impl<'a> Parser<'a> {
    fn new(i: &'a mut Iterator<Item = String>) -> Self {
        Parser {tokens: i}
    }
    fn take<T: FromStr>(&mut self) -> T {
        match self.tokens.next().expect("empty iterator").parse() {
            Ok(x) => x,
            Err(_) => panic!()
        }
    }

    fn take_some<T: FromStr>(&mut self, n: usize) -> Vec<T> {
        self.tokens.take(n).map(|s| match s.parse() { Ok(x) => x, Err(_) => panic!() } ).collect()
    }
}

//sieve[k] is true if k is a prime
//[0, n)
#[allow(dead_code)]
fn sieve_of_eratosthenes(n: u64) -> Vec<bool> {
    let mut v = vec![true; n as usize];
    v[0] = false;
    v[1] = false;
    for i in 2..n {
        if !v[i as usize] {
            continue;
        }
        let mut j = 2*i;
        while j < n {
            v[j as usize] = false;
            j += i;
        }
    }
    v
}

//[l, u)の素数
#[allow(dead_code)]
fn primes_in(l: u64, u: u64) -> Vec<u64> {
    sieve_of_eratosthenes(u).into_iter().zip(0..u).filter(|&x| x.0).map(|x| x.1).filter(|x| *x >= l).collect()
}


#[allow(dead_code)]
fn vec_2d<T>(n: usize, m: usize, t: T) -> Vec<Vec<T>> where T: Clone {
    let mut u = Vec::new();
    let mut v = Vec::new();
    v.resize(m, t);
    u.resize(n, v);
    u
}

fn test(v: &mut Vec<Vec<i32>>, i: usize, j: usize, k: usize, l: usize, a: i32) -> bool {
    for &(dx, dy) in [(1, 0), (-1, 0), (0, 1), (0, -1)].iter() {
        let (x, y) = (i as i32 + dx, j as i32 + dy);
        if x < 0 || y < 0 || x >= v.len() as i32 || y >= v[0].len() as i32 {
            continue;
        }
        let x = x as usize;
        let y = y as usize;
        if x == k && y == l {
            continue;
        }
        if v[x][y] == -1 {
            return true;
        }
        if v[x][y] == a {
            v[x][y] = -1;
            if test(v, x, y, i, j, a) {
                return true;
            }
        }
    }
    false
}

fn main() {
    let stdin = io::stdin();
    let mut tokens = stdin.lock().lines().filter_map(|x| x.ok()).flat_map(|x| x.split_whitespace().map(|s| s.to_owned()).collect::<Vec<String>>());
    let mut parser = Parser::new(&mut tokens);

    let w: usize = parser.take();
    let h: usize = parser.take();

    let mut m = Vec::new();
    for _ in 0..h {
        m.push(parser.take_some::<i32>(w));
    }
    let mut f = false;
    for i in 0..h {
        for j in 0..w {
            let mut v = m.clone();
            let a = v[i][j];
            v[i][j] = -1;
            f |= test(&mut v, i, j, i, j, a);
        }
    }
    println!("{}", if f {"possible"} else {"impossible"});
}