import sys
import threading
# threading.stack_size(4096 * 1024 * 16)
sys.setrecursionlimit(100000)

w, h = map(int, input().split())
a = [[*map(int, input().split())] for _ in range(h)]

INF = 10**9

visit = [[False for i in range(w)] for j in range(h)]
dist = [[INF for i in range(w)] for j in range(h)]
dy = [-1, 0, 1, 0]
dx = [0, 1, 0, -1]
cnt = 0


def dfs(now, pre, num):
    ret = False
    d = dist[now[0]][now[1]]
    for i in range(4):
        y2 = now[0] + dy[i]
        x2 = now[1] + dx[i]
        if y2 < 0 or y2 > h-1 or x2 < 0 or x2 > w-1:
            continue
        if not a[y2][x2] == num or [y2, x2] == pre:
            continue

        if dist[y2][x2] == INF:
            dist[y2][x2] = d + 1
            ret = dfs([y2, x2], now, num)
            if ret:
                return ret
        elif d >= 3:
            return True

    return ret


def solve():
    ret = False
    for i in range(h):
        for j in range(w):
            if dist[i][j] == INF:
                dist[i][j] = 0
                ret = dfs([i, j], [-1, -1], a[i][j])
                if ret:
                    print("possible")
                    return

    print("impossible")


# threading.Thread(target=solve).start()
solve()