There are 52 cards on a table and numbered in order from 1 to 52 with number 1 on top. The following operation can be repeatedly done: split the pile into two piles without shuffling and take cards from the top of the two piles in any order and build one new pile from top to bottom.
Example for a deck with 10 cards: given pile 1,2,3,4,5,6,7,8,9,10 make piles 1,2,3,4 and 5,6,7,8,9,10 and build a new pile such as 5,6,1,7,8,9,2,3,4,10.
What is the minimum number of operations, which is needed to revert the original pile, i.e. from 1,2,3,...,50,51,52 to 52,51,50,...,3,2,1?