from itertools import *
from bisect import *
#from math import *
from collections import *
from heapq import *
from random import *
from decimal import *
import numpy as np
import sys

sys.setrecursionlimit(10 ** 6)
int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def MI1(): return map(int1, sys.stdin.readline().split())
def MF(): return map(float, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LF(): return list(map(float, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
dij = [(1, 0), (0, 1), (-1, 0), (0, -1)]

def main():
    def dfs(i,j,pi=-1,pj=-1):
        now=t[i][j]
        t[i][j]=-now
        for di,dj in dij:
            ni,nj=i+di,j+dj
            if ni<0 or ni>=h or nj<0 or nj>=w:continue
            if (ni,nj)==(pi,pj):continue
            if t[ni][nj]==-now:return True
            if t[ni][nj]!=now:continue
            if dfs(ni,nj,i,j):return True
        return False

    w,h=MI()
    t=[LI() for _ in range(h)]
    for i in range(h):
        for j in range(w):
            if t[i][j]<0:continue
            if dfs(i,j):
                print("possible")
                exit()
    print("impossible")

main()