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