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