void draw_line()
{
    for (int i = 0; i < pos.size(); i++)
    {
        for (int j = 0; j < pos.size(); j++)
        {
            if (a[i][j])
            {
                sf::VertexArray line(sf::Lines);
                line[0].position = pos[i];
                line[0].color = sf::Color::Black;
                line[1].position = pos[j];
                line[1].color = sf::Color::Black;
                lines.push_back(line);
            }
        }
    }
}