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