#!/usr/bin/env python3.8
# %%
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
import itertools

# %%
W, H = map(int, readline().split())
M = tuple(map(int, read().split()))

# %%
N = W * H
graph = [[] for _ in range(N)]
for i, j in itertools.product(range(W - 1), range(H)):
    x = W * j + i
    if M[x] == M[x + 1]:
        graph[x].append(x + 1)
        graph[x + 1].append(x)
for i, j in itertools.product(range(W), range(H - 1)):
    x = W * j + i
    if M[x] == M[x + W]:
        graph[x].append(x + W)
        graph[x + W].append(x)


# %%
visited = [False] * N
found = False
for i in range(N):
    if visited[i]:
        continue
    visited[i] = True
    V = 1
    E = 0
    stack = [i]
    while stack:
        v = stack.pop()
        E += len(graph[v])
        for w in graph[v]:
            if visited[w]:
                continue
            V += 1
            visited[w] = True
            stack.append(w)
    E //= 2
    if V <= E:
        found = True
        break

print('possible' if found else 'impossible')