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