# -*- coding: utf-8 -*-
W, H = map(int, raw_input().split())
matA = [[0 for i in range(0, W)] for j in range(0, H)]
for i in range(0, H):
    tmp = map(int, (raw_input().split()))
    for j in range(0, W):
        matA[i][j] = tmp[j]
matB = [[0 for i in range(0, W)] for j in range(0, H)]
flag = 0
delx = (0, 1, -1, 0, 0)
dely = (0, 0, 0, 1, -1)
delm = (0, 2, 1, 4, 3)
staci = []
stacj = []
stacd = []
cnt = 0
num = 0
for i in range(0, H):
    for j in range(0, W):
        if matB[i][j] == 0:
            cnt += 1
            num = matA[i][j]
            staci.append(i)
            stacj.append(j)
            stacd.append(0)
            while len(staci) != 0:
                ii = staci.pop()
                jj = stacj.pop()
                di = stacd.pop()
                matB[ii][jj] = cnt
                for k in range(1, 5):
                    iii = ii + dely[k]
                    jjj = jj + delx[k]
                    if 0 <= iii < H and 0 <= jjj < W:
                        if matA[ii][jj] == matA[iii][jjj] and k != delm[di]:
                            staci.append(iii)
                            stacj.append(jjj)
                            stacd.append(k)
                        if matB[ii][jj] == matB[iii][jjj] and k != delm[di]:
                            print "possible"
                            exit()
print "impossible"