import sys
input=lambda: sys.stdin.readline().rstrip()
w,h=map(int,input().split())
n=w*h
edge=[[] for _ in range(n)]
M=[[int(i) for i in input().split()] for _ in range(h)]
d=((1,0),(0,1),(-1,0),(0,-1))
E=[0]*1001
for i in range(n):
  ix,iy=i//w,i%w
  for dx,dy in d:
    if 0<=ix+dx<h and 0<=iy+dy<w:
      if M[ix][iy]==M[ix+dx][iy+dy]:
        edge[i].append((ix+dx)*w+(iy+dy))
        E[M[ix][iy]]+=1
ans=False
for i in range(n):
  ix,iy=i//w,i%w
  if E[M[ix][iy]]<8 or len(edge[i])<2:
    continue
  else:
    Chk=[False]*n
    Prev=[None]*n
    Chk[i]=True
    S=[]
    S.append(i)
    while S:
      s=S.pop()
      for g in edge[s]:
        if Chk[g] and g!=Prev[s]:
          ans=True
          break
        elif not Chk[g]:
          Prev[g]=s
          S.append(g)
          Chk[g]=True
        else:
          continue
  if ans==True:
    break
if ans:
  print("possible")
else:
  print("impossible")