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Two players decide that one will try to place numbers in the Blue and Green squares and the other will try to fill the Yellow and Red ones. You must always choose diagonally opposite colours.
The central row and column are filled automatically.
After tossing a coin to decide who goes first, the players take turns to place a number in one of their squares, or take one out. The player who fills both of their squares first is the winner.
Right from the first move each player can position their numbers in a way that restricts the choices available to their opponent. Remember that if a player finds that they can't place a number in a square they have to waste moves by taking a number out and putting another in.
So, a player can deliberately try to restrict the choices of their opponent so that they can gain an advantage
See the next section.........