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