import sys
#sys.setrecursionlimit(n)
import heapq
import re
import bisect
import random
import math
import itertools
from collections import defaultdict, deque
from copy import deepcopy

w, h = map(int, input().split())
m = [[int(i) for i in input().split()] for i in range(h)] 
d = [[0 for _ in range(w)] for _ in range(h)]

dist = ((1,0), (-1,0), (0,1), (0,-1))
def solve(x, y):
  q = deque()
  q.append((x << 32) + (y << 16) + (0xFF << 8) + 0xFF)
  d[y][x] = True
  while (q):
    v = q.popleft()
    vx = v >> 32
    vy = (v >> 16) & 0xFF
    ox = (v >> 8) & 0xFF
    oy = v & 0xFF
    for fdist in dist:
      ny = vy + fdist[0]
      nx = vx + fdist[1]
      if nx == ox and ny == oy:
        continue
      if nx >= w or nx < 0 or ny >= h or ny < 0:
        continue
      if m[ny][nx] != m[y][x]:
        continue
      if d[ny][nx]:
        return True
      d[ny][nx] = True
      q.append((nx << 32) + (ny << 16) + (vx << 8) + vy)
  return False

flag = False
for y in range(h):
  for x in range(w):
    if d[y][x]:
      continue
    if solve(x, y):
      flag = True
      break

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