#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  if (n % 2 == 0)
    cout << (n * n) / 4 << endl;
  else
    cout << (n / 2) * (n + 1) / 2 << endl;
  for (int i = 1; i <= (n / 2); i++)
    for (int j = (n / 2) + 1; j <= n; j++) cout << i << " " << j << endl;
}
