Every day, Patrick the dog chews one slipper from the available stock in the house. Strictly with a probability of 0.5 Patrick wants to chew the left slipper, and with a probability of 0.5 – the right one. If the desired slippers are not present, Patrick becomes upset. How many pairs of the same slippers need to be bought, so that with a probability of not less than 0.8 Patrick does not get upset for an entire week (7 days)?