Problem Perfect Permutation

Link: http://codeforces.com/problemset/problem/233/A

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 exceed n. 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. A perfect permutation is such permutation p that for any i (1 ≤ i ≤ n) (n is the permutation size) the following equations hold p_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 

Bài toán rất đơn giản:
code here: đã AC:

#include<iostream>
using namespace std;

int main()
{
int N;
cin >> N;
if(N==1||(N%2)==1)
{
cout << -1 << endl;
// system("pause");
return 0;
}

for(int j = 1; j <= N;j+=2)
cout << j+1 << " " << j << " ";
cout << endl;
//system("pause");
return 0;

}