#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  scanf("%d", &n);
  if (n % 2 == 0) {
    cout << n / 2 * n / 2 << "\n";
  } else {
    cout << (n / 2) * (n + 1) / 2 << "\n";
  }
  for (int i = 1; i <= n / 2; ++i) {
    for (int j = n / 2 + 1; j <= n; ++j) {
      cout << i << " " << j << "\n";
    }
  }
  return 0;
}
