class UnionFind:
    def __init__(self, n):
        self.parent = list(range(n)) #親ノード
        self.size = [1]*n #グループの要素数
 
    def root(self, x): #root(x): xの根ノードを返す．
        while self.parent[x] != x:
            self.parent[x] = self.parent[self.parent[x]]
            x = self.parent[x]
        return x 
 
    def merge(self, x, y): #merge(x,y): xのいる組とyのいる組をまとめる
        x, y = self.root(x), self.root(y)
        if x == y: return False
        if self.size[x] < self.size[y]: x,y=y,x #xの要素数が大きいように
        self.size[x] += self.size[y] #xの要素数を更新
        self.parent[y] = x #yをxにつなぐ
        return True
 
    def issame(self, x, y): #same(x,y): xとyが同じ組ならTrue
        return self.root(x) == self.root(y)
        
    def getsize(self,x): #size(x): xのいるグループの要素数を返す
        return self.size[self.root(x)]
        

# coding: utf-8
# Your code here!
import sys
readline = sys.stdin.readline
read = sys.stdin.read

#a,b,c = map(int,readline().split())


w,h = map(int,input().split())
b = [list(map(int,input().split())) for _ in range(h)]
UF = UnionFind(w*h)
ok = 0
for i in range(h):
    for j in range(w):
        v = i*w+j
        if i+1 < h and b[i][j]==b[i+1][j]:
            if not UF.merge(v,v+w): ok = 1
        if j+1 < w and b[i][j]==b[i][j+1]:
            if not UF.merge(v,v+1): ok = 1

print("possible" if ok else "impossible")