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