#include <bits/stdc++.h>
const int oo = 0x3FFFFFFF;
using namespace std;
int main() {
  int n;
  scanf("%d", &n);
  if (n % 2 == 1) {
    printf("-1\n");
    return 0;
  }
  for (int i = 1; i <= n; i += 2) {
    printf("%d %d ", i + 1, i);
  }
  printf("\n");
  return 0;
}
