I have the following loop and don't understand why it only runs once:

std::vector<sf::RectangleShape> shapes;

const int res_width = 640;
const int res_height = 480;

for (int x = 0; x < res_width / 50; x += 50)
{
    for (int y = 0; y < res_height / 50; y += 50)
    {
        sf::RectangleShape shape(sf::Vector2f(50, 50));
        shape.setPosition(x * 50, y * 50);
        sf::Color color = (x % 2 == 0) ? sf::Color::Green : sf::Color::Red;
        shape.setFillColor(sf::Color::Green);
        shapes.push_back(shape);
    }
}