小李的袜子筐里有12只脏袜子和20只干净袜子．这天，他迷迷糊糊地从筐中拿袜子，每次拿两只．如果其中有脏袜子就会把两只都扔到地上，然后从筐中重拿．那么，他至少拿______次才能保证一定拿到两只干净的袜子．

If there is only an expiry date in the L/C without a shipment date，the expiry date is regarded as the latest shipment date．( )