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EXAMPLE
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a(1)=1, since the only closed route for n=1 can be considered two jumps of a chess knight from any cell and back;
a(2)=3 because the chess knight has 3 closed routes consisting of 4 jumps. We list them in the coordinate plane Oxy:
1st route: (0;0), (1;2), (3;3), (2;1), (0;0);
2nd route: (0;1), (1;3), (3;2), (2;0), (0;1);
3rd route: (0;1), (2;2), (4;1), (2;0), (0;1).
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