//#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
//using namespace atcoder;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<ll>>;
Graph G;
vll dist;
vector<bool> seen;
ll mod = 1e9 + 7;




int main() {
   ll N;
   cin>>N;
   ll k=0;
   for(ll i=1;i<N;i++){
       for(ll j=i+1;j<=N-i+1;j++){
           k++;
           
       }
   }
   cout<<k<<endl;
   for(ll i=1;i<N;i++){
       for(ll j=i+1;j<=N-i+1;j++){
           
           cout<<i<<" "<<j<<endl;
       }
   }
}