#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;

using isize = size_t;
using i32 = int;
using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;
using i128 = __int128_t;
using u128 = __uint128_t;
using f64 = long double;
using p2 = pair<i64, i64>;
using el = tuple<i64, i64, i64>;
using mint = atcoder::modint998244353;

void _main();
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(18);
  _main();
}

i64 pow(i64 x, i64 n) {
  i64 res = 1;
  i64 t = x;
  while (n > 0) {
    if (n & 1) {
      res = res * t;
    }
    t = t * t;
    n >>= 1;
  }
  return res;
}
i64 pow(i64 x, i64 n, i64 m) {
  i64 res = 1;
  i64 t = x % m;
  while (n > 0) {
    if (n & 1) {
      res = res * t % m;
    }
    t = t * t % m;
    n >>= 1;
  }
  return res;
}

void _main() {
  i64 n;
  cin >> n;
  if (n <= 2) {
    cout << "-1\n";
    return;
  }
  vector<p2> g;
  for (i64 i = 0; i < n; i++) {
    for (i64 j = 0; j < n; j++) {
      g.push_back({i * n + j, i * n + (j + 1) % n});
    }
  }
  for (i64 i = 0; i < n; i++) {
    for (i64 j = 0; j < n; j++) {
      g.push_back({i * n + j, (i + 1) % n * n + j});
    }
  }
  cout << g.size() << "\n";
  for (auto [a, b] : g) {
    cout << a + 1 << " " << b + 1 << "\n";
  }
}