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 | | If this configuration were obtainable starting from the permutation [3,5,2,4,1] (those with 7 passing considered at the beginning), it would mean that, starting from number 7 and adding or subtracting only even numbers we are able to get 0. |
 | | If for a permutation the number of passing is even, we say that the permutation is of even class (or simply even), otherwise is of odd class (or odd). |
 | | For instance the permutation [3,5,2,4,1] is an odd permutation, while the permutation [3,5,4,2,1] is even. |
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