#include <bits/stdc++.h>
using namespace std;
long long int inf = 1e9;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  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;
    }
  }
  return 0;
}
