#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  cout << (n * n) / 4 << endl;
  for (int i = 1; i <= n / 2; i++) cout << n << ' ' << i << endl;
  for (int i = 1; i <= n / 2; i++)
    for (int j = n / 2 + 1; j < n; j++) cout << i << ' ' << j << endl;
}
