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