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