import sys
sys.setrecursionlimit(12000)

# Input
W, H = map(int, input().split())
A = [ list(map(int, input().split())) for i in range(H) ]

# DFS Function
def dfs(x, y, A, vis):
	dx = [ 1, 0, -1, 0 ]
	dy = [ 0, 1, 0, -1 ]
	vis[x][y] = True
	cnts = (1, 0)
	for i in range(4):
		tx, ty = x + dx[i], y + dy[i]
		if 0 <= tx and tx < H and 0 <= ty and ty < W and A[tx][ty] == A[x][y]:
			if not vis[tx][ty]:
				res = dfs(tx, ty, A, vis)
				cnts = (cnts[0] + res[0], cnts[1] + res[1])
			cnts = (cnts[0], cnts[1] + 1)
	return cnts
	

# DFS
vis = [ [ False ] * W for i in range(H) ]
answer = False
for i in range(H):
	for j in range(W):
		if not vis[i][j]:
			res = dfs(i, j, A, vis)
			if res[1] // 2 != res[0] - 1:
				answer = True

# Output
if answer:
	print("possible")
else:
	print("impossible")