#yuki_13

def dfs(y,x,py,px,n):
 global suc
 if suc: return
 done[y][x]=True
 for i in xrange(4):
  ny=y+dy[i]
  nx=x+dx[i]
  if not (0<=ny and ny<H and 0<=nx and nx<W): continue
  if ny==py and nx==px: continue
  if a[ny][nx]!=n: continue
  if done[ny][nx]:
   sucue
   return
  dfs(ny,nx,y,x,n)


W,H=map(int,raw_input().split())
a=[]
for i in xrange(H):
 temp=map(int,raw_input().split())
 a.append(temp)
done=[[0 for i in xrange(W)] for j in xrange(H)]
dx=[-1,0,1,0]
dy=[0,1,0,-1]
suc=False

for i in xrange(H):
 for j in xrange(W):
  if suc: break
  if done[i][j]: continue
  dfs(i,j,i,j,a[i][j])
print 'possible' if suc else 'impossible'