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