import collections

directions = ((-1, 0), (0, -1), (1, 0), (0, 1))

def bfs(sx, sy, w, h, field, visited):
    queue = collections.deque()
    queue.append((sx, sy, -1, -1))
    visited[sy][sx] = True

    while queue:
        x, y, px, py = queue.popleft()

        for dx, dy in directions:
            nx = x + dx
            ny = y + dy

            if (not (0 <= nx < w and 0 <= ny < h) or
                nx == px and ny == py or
                field[y][x] != field[ny][nx]):
                continue

            if visited[ny][nx]:
                return True

            visited[ny][nx] = True
            queue.append((nx, ny, x, y))

    return False

def solve():
    w, h = map(int, raw_input().split())
    field = [map(int, raw_input().split()) for _ in xrange(h)]
    visited = [[False] * w for _ in xrange(h)]

    for y in xrange(h):
        for x in xrange(w):
            if visited[y][x]:
                continue

            if bfs(x, y, w, h, field, visited):
                return True

    return False

print 'possible' if solve() else 'impossible'