#include <bits/stdc++.h>   using namespace std;   int main() {     int n;     cin >> n;     if (n % 2 != 0)       cout <<  -1  << endl;     else {       for (int i = 1; i <= n; i++) {         if (i % 2 == 0)           cout << i - 1 <<    ;         else           cout << i + 1 <<    ;       }     }     return 0;   }   