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