#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimization("O3")
#pragma GCC optimization("unroll-loops")
#pragma GCC target( \
    "sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,fma,tune=native")
#pragma GCC optimize("-ffloat-store")
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  if (n % 2)
    cout << -1 << "\n";
  else {
    for (int i = 1; i <= n; i++) {
      cout << i + 1 << " " << i << " ";
      i++;
    }
  }
  return 0;
}
