W, H = map(int, input().split())

field = []
for _ in range(H):
    field.append(list(map(int, input().split())))

def ok(x, y):
    return y >= 0 and x >= 0 and x < H and y < W

def dfs(x, y):
    dx = [1, 0, -1, 0]
    dy = [0, 1, 0, -1]
    visit[x][y] = True
    c = (1, 0)

    for k in range(4):
        ny = y + dy[k]
        nx = x + dx[k]
        if ok(nx, ny) and field[nx][ny] == field[x][y]:
            if not visit[nx][ny]:
                res = dfs(nx, ny)
                c = (c[0] + res[0], c[1] + res[1])
            c = (c[0], c[1] + 1)

    return c

visit = [[False for _ in range(W)] for _ in range(H)]
ans = False

for i in range(H):
    for j in range(W):
        if not visit[i][j]:
            res = dfs(i, j)
            if res[1] // 2 != res[0] - 1:
                ans = True

if ans:
    print("possible")
else:
    print("impossible")