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]

for a in range(H):
   for b in range(W):
      arrived=[[False for i in range(W)] for j in range(H)]
      st = [(a,b,1)]
      while len(st)!=0:
         i,j,dist = 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 a== ni and b == nj and dist >= 3:
                     print 'possible'
                     exit()
                  elif not arrived[ni][nj]:
                     st.append((ni,nj,dist+1))
print 'impossible'