import bisect
import copy
import decimal
import fractions
import heapq
import itertools
import math
import random
import sys
from collections import Counter, deque,defaultdict
from functools import lru_cache,reduce
from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max
def _heappush_max(heap,item):
    heap.append(item)
    heapq._siftdown_max(heap, 0, len(heap)-1)
def _heappushpop_max(heap, item):
    if heap and item < heap[0]:
        item, heap[0] = heap[0], item
        heapq._siftup_max(heap, 0)
    return item
from math import gcd as GCD
read=sys.stdin.read
readline=sys.stdin.readline
readlines=sys.stdin.readlines

class UnionFind:
    def __init__(self,n):
        self.n=n
        self.parents=[-1]*n

    def Find(self,x):
        stack=[]
        while self.parents[x]>=0:
            stack.append(x)
            x=self.parents[x]
        for y in stack:
            self.parents[y]=x
        return x

    def Union(self,x,y):
        x=self.Find(x)
        y=self.Find(y)
        if x==y:
            return
        if self.parents[x]>self.parents[y]:
            x,y=y,x
        self.parents[x]+=self.parents[y]
        self.parents[y]=x

    def Size(self,x):
        return -self.parents[self.Find(x)]

    def Same(self,x,y):
        return self.Find(x)==self.Find(y)

    def Members(self,x):
        root = self.Find(x)
        return [i for i in range(self.n) if self.Find(i)==root]

    def Roots(self):
        return [i for i, x in enumerate(self.parents) if x<0]

    def Group_Count(self):
        return len(self.Roots())

    def All_Group_Members(self):
        group_members = defaultdict(list)
        for member in range(self.n):
            group_members[self.Find(member)].append(member)
        return group_members

    def __str__(self):
        return '\n'.join(f'{r}: {m}' for r, m in self.All_Group_Members().items())

W,H=map(int,readline().split())
M=[list(map(int,readline().split())) for i in range(H)]
UF=UnionFind(H*W)
ans='impossible'
for i in range(H):
    for j in range(W-1):
        if M[i][j]!=M[i][j+1]:
            continue
        if UF.Same(i*W+j,i*W+j+1):
            ans='possible'
        UF.Union(i*W+j,i*W+j+1)
for i in range(H-1):
    for j in range(W):
        if M[i][j]!=M[i+1][j]:
            continue
        if UF.Same(i*W+j,(i+1)*W+j):
            ans='possible'
        UF.Union(i*W+j,(i+1)*W+j)
print(ans)