A permutation is a sequence of integers p1,?p2,?...,?pn, consisting of n distinct positive integers, each of them doesn't exceed n. Let's denote the i-th element of permutation p as pi. We'll call number n the size of permutation p1,?p2,?...,?pn.
Nickolas adores permutations. He likes some permutations more than the others. He calls such permutations perfect. A perfectpermutation is such permutation p that for any i (1?≤?i?≤?n) (n is the permutation size) the following equations hold ppi?=?i and pi?≠?i. Nickolas asks you to print any perfect permutation of size n for the given n.
A single line contains a single integer n (1?≤?n?≤?100) ― the permutation size.
If a perfect permutation of size n doesn't exist, print a single integer ⑴. Otherwise print n distinct integers from 1 to n, p1,?p2,?...,?pn ― permutation p, that is perfect. Separate printed numbers by whitespaces.
解題思路：貪心。直接構造。如果n為奇數，則不能構成滿足要求的排列；否則，兩個兩個的構造，如2,1；4,3。。。以此類推便可。
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