/* -*- coding: utf-8 -*-
 *
 * 1519.cc:  No.1519 Diversity - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 500;
const int MAX_M = MAX_N * (MAX_N - 1) / 2;

/* typedef */

typedef pair<int,int> pii;

/* global variables */

pii es[MAX_M];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);

  int m = 0;
  for (int i = 0, j = n - 1; i < j; i++, j--)
    for (int k = i + 1; k <= j; k++) es[m++] = pii(i, k);

  printf("%d\n", m);
  for (int i = 0; i < m; i++)
    printf("%d %d\n", es[i].first + 1, es[i].second + 1);
  return 0;
}