W,H=map(int,raw_input().split())
M=[]
for i in range(H):
   Ms = map(int,raw_input().split())
   M.append(Ms)


dx=[1,0,-1,0]
dy=[0,1,0,-1]

arrived=[[False for i in range(W)] for j in range(H)]
for a in range(H):
   for b in range(W):
      st = [(a,b,-1,-1)]
      while len(st)!=0:
         i,j,pi,pj = st.pop()
         if arrived[i][j]:
            continue
         arrived[i][j]=True
         for x,y in zip(dx,dy):
            ni = i+x
            nj = j+y
            if ni >= 0 and ni < H and nj >= 0 and nj <W:
               if M[ni][nj]==M[i][j]:
                  if not arrived[ni][nj]:
                     st.append((ni,nj,i,j))
                  elif pi!=-1:
                     if (ni,nj)!=(i,j) and (ni,nj)!=(pi,pj):
                        print 'possible'
                        exit()
print 'impossible'