#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++)
    for (int j = n / 2 + 1; j <= n; j++) cout << i << " " << j << endl;
  return 0;
}
