import sys
input = sys.stdin.buffer.readline
sys.setrecursionlimit(10 ** 7)
DX = (-1, 0, 1, 0, -1, -1, 1, 1)
DY = (0, 1, 0, -1, -1, 1, -1, 1)
DX = DX[:4]
DY = DY[:4]

W, H = map(int, input().split())
G = tuple(tuple(map(int, input().split())) for _ in range(H))
vis = [[0]*W for _ in range(H)]


def dfs(num, x, y, px, py):
    vis[x][y] = 1
    for dx, dy in zip(DX, DY):
        nx = x + dx
        ny = y + dy
        if (nx, ny) == (px, py):
            continue
        if 0 <= nx < H and 0 <= ny < W and G[nx][ny] == num:
            if vis[nx][ny]:
                return True
            ok = dfs(num, nx, ny, x, y)
            if ok:
                return True
    vis[x][y] = 0
    return False


used = set()
for i in range(H):
    for j, v in enumerate(G[i]):
        if v in used:
            continue
        if dfs(v, i, j, -1, -1):
            print("possible")
            exit()
        used.add(v)

print("impossible")