For the buffet, the waiter of a restaurant gets a large stack of pancakes from the overworked cook. As expected under such harsh work conditions, one side is burnt, and as the level of batter decreases, the pancakes became smaller and smaller. Hence, the waiter ends up with a stack of one-sided burnt pancakes sorted by size, with the larger at the bottom and burnt side up. However, the waiter cannot serve them this way. He needs to turn all the burnt sides down, without changing the order. Having only a spatula, he can only perform flips to the top of the stack. How can he perform this transformation in a minimum number of flips?