import sys
from collections import deque
sys.setrecursionlimit(10**7)
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))
def LI2(): return list(map(int,sys.stdin.readline().rstrip()))
def S(): return sys.stdin.readline().rstrip()
def LS(): return list(sys.stdin.readline().rstrip().split())
def LS2(): return list(sys.stdin.readline().rstrip())


W,H = MI()
M = [LI() for _ in range(H)]

flag = [[0]*W for _ in range(H)]
directions = [(-1,0),(1,0),(0,-1),(0,1)]
for i in range(H):
    for j in range(W):
        if flag[i][j] == 0:
            flag[i][j] = 1
            deq = deque([(i,j,-1,-1)])
            while deq:
                ci,cj,pi,pj = deq.pop()
                for di,dj in directions:
                    ni,nj = ci+di,cj+dj
                    if not (0 <= ni < H and 0 <= nj < W) or M[ni][nj] != M[i][j] or (ni,nj) == (pi,pj):
                        continue
                    elif flag[ni][nj] == 0:
                        deq.append((ni,nj,ci,cj))
                        flag[ni][nj] = 1
                    else:
                        print('possible')
                        exit()

print('impossible')