Perfect Permutation

Description
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) (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.

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, …, 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
C语言版本一

#include <stdio.h>
#include <stdlib.h>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */

int main(int argc, char *argv[]) {
	int n;
	scanf("%d",&n);
	int i;
	if(n%2!=0){
		printf("-1\n");
	}else{
		if(n>2){
			
			for(i=1;i<=n-2;i++){
				printf("%d ",i+1);
				printf("%d ",i);
				i++;
			}
			
		}
			printf("%d ",n);
			printf("%d\n",n-1);
		
	}
	
	return 0;
}