w, h = map(int, input().split())
m = [list(map(int, input().split())) for _ in range(h)]
pre = [[None] * w for _ in range(h)]
ok = False
for i in range(h):
    for j in range(w):
        if pre[i][j] is None:
            num = m[i][j]
            pre[i][j] = i, j
            q = [(i, j)]
            while q:
                x, y = q.pop()
                for nx, ny in (x-1, y), (x+1, y), (x, y-1), (x, y+1):
                    if 0 <= nx < h and 0 <= ny < w and m[nx][ny] == num and (nx, ny) != pre[x][y]:
                        if pre[nx][ny] is None:
                            pre[nx][ny] = x, y
                            q.append((nx, ny))
                        else:
                            ok = True
print('im' * (not ok) + 'possible')