A. Perfect Permutation

time limit per test

2 seconds

memory limit per test

256 megabytes

input

standard input

output

standard output

A permutation is a sequence of integers p₁, p₂, ..., p_n, consisting of n distinct positive integers, each of them doesn't exceedn. Let's denote the i-th element of permutation p as p_i. We'll call number n the size of permutation p₁, p₂, ..., p_n.

Nickolas adores permutations. He likes some permutations more than the others. He calls such permutations perfect. Aperfect permutation is such permutation p that for any i (1 ≤ i ≤ n) (n is the permutation size) the following equations holdp_pi = i and p_i ≠ 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 n distinct integers from 1 to n,p₁, p₂, ..., p_n — permutation p, that is perfect. Separate printed numbers by whitespaces.

Sample test(s)

input

1

output

-1

input

2

output

2 1 

input

4

output

2 1 4 3 

解题思路：此题就看给的样例，发现n为奇数的时候不可以，偶数的时候只需要每两个互换位置即可

#include <iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
using namespace std;

int main()
{
	int n;
	int i;
	scanf("%d",&n);
	if(n%2==1)
	{
		printf("-1\n");
	}
	else
	{
		for(i=1;i<=n/2;i++)
		{
			printf("%d %d ",2*i,2*i-1);
		}
		printf("\n");
	}
    return 0;
}