fun main(): u0 {
    var a = 1;
    var b = 0;
    var previous = 0;
    draw_number(0, 24, 13);

    while(a) {
        previous = b;
        b = get_key();
        draw_number(previous, 24, 13);
        draw_number(b, 24, 13);
    }
}