/*#include <iostream> // cout, endl, cin
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <cstdint> // int64_t, int*_t
#include <cstdio> // printf
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque
#include <unordered_map> // unordered_map
#include <unordered_set> // unordered_set
#include <bitset> // bitset
#include <cctype> // isupper, islower, isdigit, toupper, tolower
#include <iomanip>//fixed,setprecision
#include <limits.n>//INT_MAX
#include <math.n>//M_PI
#include <random>
#include <time.h>*/
#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using mint = modint1000000007;
//using mint = modint998244353;
std::chrono::time_point<std::chrono::steady_clock> start;
template <typename T> bool chmax(T &u, const T z) { if (u < z) {u = z; return true;} else return false; }
template <typename T> bool chmin(T &u, const T z) { if (u > z) {u = z; return true;} else return false; }
#define ll long long
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
typedef pair<ll, int> P;
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
ll lcm(ll a, ll b){return a * b / gcd(a, b);}

int main(){
  int n;
  cin>>n;
  vector<pair<int,int>>a;
  int cnt=0;
  for(int j=0;j<=n-1;j++){
    for (int i=j+1;i<n-j;i++){
      a.emplace_back(i,n-j);
      cnt++;
    }
  }
  cout<<cnt<<endl;
  rep(i,a.size())cout<<a[i].first<<" "<<a[i].second<<endl;
  return 0;
}