#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <unordered_set>
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int,int> > vpii;
typedef vector<vector<int> > vvi;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<ll> > vvll;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n) - 1; i >= 0; --i)
#define fin(ans) cout<<(ans)<<endl
#define STI(s) atoi(s.c_str())
#define mp(p,q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define Rort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
#define itn int
template <class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template <class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}
struct io{io(){ios::sync_with_stdio(false);cin.tie(0);};};
const int MOD = 1000000007;
const int INF = INT_MAX;
const ll LLINF = 1LL<<62;



int main(void) {

    int n;
    cin>>n;
    vector<vector<int> > allG(n);
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            if(i==j)continue;
            allG[i].push_back(j);
        }
    }
    vi allsum(n);
    for(int i=0;i<n;i++){
        allsum[i]=n*(n+1)/2-(i+1);
    }
    for(int s=n*n;s>=0;s--){
        // cout<<s<<endl;
        vvi chG = allG;
        vi sumG = allsum;
        bool ch=true;
        while(ch){
            bool fin=true;
            for(int i=0;i<n;i++){
                if(sumG[i]<s){
                    ch=false;
                    break;
                }
            }
            // for(int i=0;i<n;i++){
            //     cout<<sumG[i]<<" ";
            // }
            // cout<<endl;
            int maxpos=0;
            int maxval=0;
            int reduce=0;
            for(int i=0;i<n;i++){
                if(maxval<=sumG[i]){
                    maxval=sumG[i];
                    maxpos=i;
                }
            }
            // cout<<"AA"<<endl;
            //rem
            bool ischange=false;
            for(int i=0;i<chG[maxpos].size();i++){
                // cout<<"i:"<<i<<endl;
                if(sumG[maxpos]-chG[maxpos][i]-1==s){
                    sumG[maxpos]-=chG[maxpos][i]+1;
                    sumG[chG[maxpos][i]]-=maxpos+1;

                    int from=maxpos+1;
                    int to=chG[maxpos][i]+1;
                    // cout<<"FT "<<from<<" "<<to<<endl;

                    chG[from-1].erase(find(all(chG[from-1]),to-1));
                    chG[to-1].erase(find(all(chG[to-1]),from-1));
                    ischange=true;
                    break;
                }
            }
            // cout<<"BB"<<endl;
            if(!ischange){
                Sort(chG[maxpos]);
                sumG[maxpos]-=chG[maxpos][0]+1;
                chG[maxpos].erase(chG[maxpos].begin());
            }
            for(int i=0;i<n;i++){
                if(sumG[i]!=s){
                    fin=false;
                    break;
                }
            }
            // for(int i=0;i<n;i++){
            //     cout<<"i: "<<i+1<<" =";
            //     for(int j=0;j<chG[i].size();j++){
            //         cout<<chG[i][j]+1<<" ";
            //     }
            //     cout<<endl;
            // }
            if(fin){
                //出力
                set<pair<int,int> > sp;
                vector<pair<int,int> > ans;
                for(int i=0;i<n;i++){
                    for(int j=0;j<chG[i].size();j++){
                        if(sp.find(mp(i+1,chG[i][j]+1))!=sp.end())continue;
                        ans.push_back(mp(i+1,chG[i][j]+1));
                        sp.insert(mp(i+1,chG[i][j]+1));
                        sp.insert(mp(chG[i][j]+1,i+1));
                    }
                }
                cout<<(int)ans.size()<<endl;
                for(int i=0;i<ans.size();i++){
                    cout<<ans[i].first<<" "<<ans[i].second<<endl;
                }
                return 0;
            }
        }
    }

}



Measured memory_bytes: 896