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