//GIVE ME AC!!!!!!!!!!!!!!!!!
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
#define ll long long
#define floatset() fixed<<setprecision(15)
#define all(n) n.begin(),n.end()
#define rall(n) n.rbegin(),n.rend()
#define rep(i, s, n) for (ll i = s; i < (ll)(n); i++)
using namespace std;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const ll inf =1e18;
const ll MOD=1000000007;
const ll mod=998244353;
const double pi=acos(-1);
using P=pair<ll,ll>;
int main(){
  ll n;
  cin>>n;
  cout<<n-1<<endl;
  rep(i,1,n+1){
    rep(j,i+1,n-i+2){
      cout<<i<<" "<<j<<endl;
    }
  }
}