w,h=map(int,input().split())
m=[list(map(int,input().split())) for i in range(h)]
v=[[0]*w for i in range(h)]
for i in range(h):
  for j in range(w):
    v=[[0]*w for x in range(h)]
    x,y=i,j
    v[x][y]=1
    q=[(x,y,x,y)]
    for x,y,xxx,yyy in q:
      dx,dy=1,0
      for _ in range(4):
        xx,yy=x+dx,y+dy
        if 0<=xx<h and 0<=yy<w:
          if m[xx][yy]==m[x][y]:
            if v[xx][yy]==0:
              v[xx][yy]=1
              q+=[(xx,yy,x,y)]
            elif (xx,yy)!=(xxx,yyy):
              print("possible")
              exit()
        dx,dy=-dy,dx
print("impossible")