#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 / 2; ++i) {
    for (int j = n / 2 + 1; j <= n; ++j) {
      printf("%d %d\n", i, j);
    }
  }
  return 0;
}
