The original text is the following. If n is odd, then it's now easy to build an example: for i from 1 to n ai=2i−1, ai=2i, if i is even, and ai=2i , ai=2i−1 if i is odd.

I think that it will be clearer if the text will be changed as the following. If n is odd, then it's now easy to build an example: for i from 1 to n, ai=2i-1, ai=2i, for i from n+1 to 2n, ai=2i, ai=2i-1.