#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 <= n; i += 2) {
    for (int j = 2; j <= n; j += 2) {
      cout << i << " " << j << endl;
    }
  }
}
