import collections
import sys
W,H = map(int,input().split())
lsM = [list(map(int,input().split())) for i in range(H)]
usedn = [[False]*(W) for i in range(H)]
before = [[(-1,-1) for j in range(W)] for i in range(H)]
dxy = [(1,0),(0,1),(-1,0),(0,-1)]
#DFSで探す閉路判定
for i in range(H):
    for j in range(W):
        if usedn[i][j]:
            continue
        d = collections.deque([(i,j)])
        n = lsM[i][j]
        while d:
            x,y = d.pop()
            if usedn[x][y]:
                continue
            usedn[x][y] = True
            for dx,dy in dxy:
                if 0 <= x+dx < H and 0 <= y+dy < W:
                    if lsM[x+dx][y+dy] != n:
                        continue
                    if usedn[x+dx][y+dy]:
                        if before[x][y] == (x+dx,y+dy):
                            continue
                        else:
                            print('possible')
                            sys.exit()
                    before[x+dx][y+dy] = (x,y)
                    d.append((x+dx,y+dy))
print('impossible')