Coin puzzle

You have 6 stones*, 3 of one colour and 3 of another colour, arranged on a card with 8 squares as shown on the left.

You have to move the stones in the top row to the bottom row and those in the bottom row to the top row.
You must move alternate stones. The stones may only be moved into an empty square. You may move forwards, backwards or sideways, but not diagonally and only one square at a time.

We are going to play with beer bottle tops today. So you have to move
a Castle top, then a Windhoek top, then a Castle top and so on.

How many moves do you need to swap the positions of the tops?

* This is a fun game to play in a bar (or on the beach) with two sets of different coins, pebbles or bottle tops.

Problem adapted from: Godfrey Hall. 1991. The Book of Mindbenders. Kingfisher Books, London.
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