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

checked=set()

def dfs(x,y):
  if (x,y) in checked: return False
  r=[(0,-1),(1,0),(-1,0),(0,1)]
  c=m[y][x]
  stack=[(-1,x,y)]
  while len(stack):
    dir,tx,ty=stack.pop()
    if m[ty][tx]==c:
      if (tx,ty) in checked: return True
      checked.add((tx,ty))
      for i,(dx,dy) in enumerate(r):
        if i!=dir:
          stack.append((3-i,tx+dx,ty+dy))

print( "possible" if any([dfs(x,y) for x in range(w) for y in range(h)]) else "impossible")