import sys; input = sys.stdin.buffer.readline
sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

class UnionFind:
	def __init__(self, n):
		self.par = [i for i in range(n + 1)]
		self.rank = [0] * (n + 1)
		self.size = [1] * (n + 1)
		self.sizeEdge = [0] * (n + 1)

	def find(self, x):
		if self.par[x] == x:
			return x
		else:
			self.par[x] = self.find(self.par[x])
			return self.par[x]

	def same_check(self, x, y):
		return self.find(x) == self.find(y)

	def union(self, x, y):
		x = self.find(x); y = self.find(y)
		if self.rank[x] < self.rank[y]:
			if self.same_check(x, y) != True:
				self.size[y] += self.size[x]
				self.size[x] = 0
				self.sizeEdge[y] += self.sizeEdge[x]
				self.sizeEdge[x] = 0

			self.par[x] = y
		else:
			if self.same_check(x, y) != True:
				self.size[x] += self.size[y]
				self.size[y] = 0
				self.sizeEdge[x] += self.sizeEdge[y]
				self.sizeEdge[y] = 0

			self.par[y] = x
			if self.rank[x] == self.rank[y]:
				self.rank[x] += 1

	def siz(self, x):
		x = self.find(x)
		return self.size[x]


#処理内容
def main():
	W, H = getlist()
	table = []
	for i in range(H):
		l = getlist()
		table.append(l)

	# UFで辺の数と頂点数を管理
	UF = UnionFind(H * W)
	for i in range(H - 1):
		for j in range(W):
			if table[i][j] == table[i + 1][j]:
				UF.sizeEdge[i * W + j] += 1
				UF.sizeEdge[(i + 1) * W + j] += 1

	for i in range(H):
		for j in range(W - 1):
			if table[i][j] == table[i][j + 1]:
				UF.sizeEdge[i * W + j] += 1
				UF.sizeEdge[i * W + j + 1] += 1

	for i in range(H - 1):
		for j in range(W):
			if table[i][j] == table[i + 1][j]:
				UF.union(i * W + j, (i + 1) * W + j)

	for i in range(H):
		for j in range(W - 1):
			if table[i][j] == table[i][j + 1]:
				UF.union(i * W + j, i * W + j + 1)

	#最後に根更新
	for i in range(H * W):
		UF.par[i] = UF.find(i)

	for i in range(H):
		for j in range(W):
			node = UF.size[i * W + j]
			edge = UF.sizeEdge[i * W + j]
			if node != 0 and edge >= 2 * node:
				print("possible")
				return

	print("impossible")


if __name__ == '__main__':
	main()