#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  while (cin >> n) {
    int num[110];
    if (n % 2 != 0) {
      cout << -1 << endl;
      continue;
    } else {
      int a, b;
      for (int i = 0, j = 0; i < n; i += 2) {
        a = i + 1, b = i + 2;
        swap(a, b);
        num[j] = a;
        j++;
        num[j] = b;
        j++;
      }
    }
    for (int i = 0; i < n; i++) cout << num[i] << " ";
    cout << endl;
  }
  return 0;
}
