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