#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  if (n % 2 != 0)
    cout << -1;
  else
    for (int i = 1; i <= n / 2; i++) cout << 2 * i << " " << 2 * i - 1 << " ";
}
