W, H = map(int, input().split())
M = [list(map(int, input().split())) for _ in range(H)]
P = [[-1] * W for _ in range(H)]
dir = [[0, 1], [1, 0], [-1, 0], [0, -1]]
for h in range(H):
    for w in range(W):
        if P[h][w] == -1:
            Q = [(h, w)]
            Q2 = []
            a = M[h][w]
            while Q:
                while Q:
                    x, y = Q.pop()
                    for xd, yd in dir:
                        x1 = x + xd
                        y1 = y + yd
                        if 0 <= x1 < H and 0 <= y1 < W:
                            if M[x1][y1] != a:
                                continue
                            if (x1,y1) == P[x][y]:
                                continue
                            if P[x1][y1] == -1:
                                P[x1][y1] = (x,y)
                                Q2.append((x1,y1))
                            else:
                                print("possible")
                                exit()
                Q,Q2 = Q2,Q
print("impossible")