#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n;
  cin >> n;
  if (n % 2 == 0)
    cout << n / 2 * n / 2 << "\n";
  else
    cout << n / 2 * (n + 1) / 2 << "\n";
  for (long long i = 1; i <= n / 2; i++) {
    for (long long j = n / 2 + 1; j <= n; j++) cout << i << " " << j << "\n";
  }
}
