from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
input = sys.stdin.readline


W,H = map(int,input().split())
M = [list(map(int,input().split())) for _ in range(H)]
def nb(x,y):
    tmp = []
    if x+1<H:
        tmp.append((x+1,y))
    if x-1>=0:
        tmp.append((x-1,y))
    if y+1<W:
        tmp.append((x,y+1))
    if y-1>=0:
        tmp.append((x,y-1))
    return tmp
visited = [[False]*W for _ in range(H)]
for i in range(H):
    for j in range(W):
        if visited[i][j]:
            continue
        
        visited[i][j] = True
        m = M[i][j]
        v = deque()
        dist = defaultdict(lambda: -1)
        par = defaultdict(lambda: -1)
        v.append((i,j))
        while v:
            
            ix,iy = v.popleft()
            
            for jx,jy in nb(ix,iy):
                if (jx,jy)==par[(ix,iy)]:
                    continue
                if M[jx][jy] != m:
                    continue
                par[(jx,jy)]=(ix,iy)
                if visited[jx][jy]:
                    print('possible')
                    exit()
                visited[jx][jy] = True
                v.append((jx,jy))
print('impossible')