#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  scanf("%d", &n);
  printf("%d\n", n / 2 * (n - n / 2));
  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;
}
