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 perfect permutation is such permutation p that for any i (1 ≤ i ≤ n) (nis 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.
Input
A single line contains a single integer n (1 ≤ n ≤ 100) — the permutation size.
Output
If a perfect permutation of size n doesn't exist, print a single integer -1. Otherwise print ndistinct integers from 1 to n, p1, p2, ..., pn — permutation p, that is perfect. Separate printed numbers by whitespaces.
Examples
Input
1
Output
-1
Input
2
Output
2 1
Input
4
Output
2 1 4 3
AC代码(水题)
#include <iostream>
#include <bits/stdc++.h>
using namespace std;
int main()
{
int n, a[120];
int i;
while(scanf("%d",&n)!=EOF)
{
if(n%2!=0)
printf("-1\n");
else
{
for(i = 1;i<=n;i++)
{
if(i%2==0)
printf("%d%c",i-1,i==n?'\n':' ');
else
printf("%d ",i+1);
}
}
}
return 0;
}