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