from collections import deque
from sys import exit

W, H = map(int, input().split())
M = [tuple(map(int, input().split())) for _ in range(H)]

table = [[None] * W for _ in range(H)]
nvisited = {(h, w) for h in range(H) for w in range(W)}


def dfs(h0, w0):
    D = deque()
    n = M[h0][w0]
    table[h0][w0] = n
    D.append((h0, w0, 0, 0))
    while D:
        h, w, dhb, dwb = D.pop()
        for dh, dw in ((1, 0), (-1, 0), (0, 1), (0, -1)):
            if dhb == -dh and dwb == -dw:
                continue
            if 0 <= h + dh < H and 0 <= w + dw < W:
                if table[h + dh][w + dw] is None and M[h + dh][w + dw] == n:
                    table[h + dh][w + dw] = n
                    nvisited.remove((h + dh, w + dw))
                    D.append((h + dh, w + dw, dh, dw))
                elif table[h + dh][w + dw] == n:
                    print('possible')
                    exit()


while nvisited:
    h, w = nvisited.pop()
    dfs(h, w)
print('impossible')