#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main() {
  ll N;
  cin >> N;
  if(N <= 2){
    cout << -1 << endl;
    return 0;
  }
  ll M = 2 * N * N;
  cout << M << endl;
  for(ll i = 1; i < N * N; i++){
    cout << i << " " << i + 1 << endl;
  }
  cout << N * N << " " << 1 << endl;
  for(ll i = 1; i <= N; i++){
    for(ll j = 0; j < N - 1; j++){
      cout << i + j * N << " " << i + (j + 1) * N << endl;
    }
    cout << i + (N - 1) * N << " " << i << endl;
  }
  return 0;
}