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