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