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