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