#include <bits/stdc++.h>
using namespace std;
int main() {
  (ios_base::sync_with_stdio(false), cin.tie(NULL));
  ;
  int n, m, k, i, p;
  cin >> n;
  if (n % 2)
    cout << -1 << endl;
  else {
    for (i = 2; i <= n; i += 2) {
      cout << i << " " << i - 1 << " ";
    }
    cout << endl;
  }
}
