#include <bits/stdc++.h>
using namespace std;
int n;
int main() {
  cin >> n;
  cout << (n * n) / 4 << endl;
  for (int i = 1; i <= (n + 1) / 2; i++)
    for (int j = (n + 1) / 2 + 1; j <= n; j++) cout << i << " " << j << endl;
  return 0;
}
