#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  if (n & 1) return cout << -1, 0;
  for (int i = 1; i <= n; i += 2) cout << i + 1 << ' ' << i << ' ';
}
