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