while(1)
{
   enemy_pan1.pos_x = minv(enemy_pan1.pos_x + speed_x, 500);
   while(enemy_pan1.pos_x != 300)
   {
      enemy_pan1.pos_x = maxv(enemy_pan1.pos_x - speed_x, 300);
      wait(1);
   }
   wait(1);
}