/*
                                                     ``.-''!!!!!!!!!!!!!!!!!!!!!!!!!!!''-..`                                                     
                                            `.'!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'.`                                            
                                     `.'!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'.`                                     
                                `.!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.`                                
                            `'!!!!!!!!!!!!!!!!!!(((*****((!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'`                            
                        `'!!!!!!!!!!!+"u%$$####################$$%3""!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'`                        
                     .!!!!!!!!!!!!!"#############$$$$$$$$$############$!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.                     
                  `!!!!!!!!!!!!!!!!!"z%%3I"+(!!!!!!!!!!!!!!!!!!*"I3%%j(!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!`                  
                -!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!-                
              '!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!(*"""""IjjjjIt""""*(!!!!!!!!!!!!!!!!!!!!'              
            '!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!(I%$$##$$%"!!!!!!!!!!!!!!!!!!!!!!!!!!!3$##########################$$3""!!!!!!!!!!!!!!'            
          .!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"$############$(!!!!!!!!!!!!!!!!!!!!!!!($######$#########$3&%$$$##########$!!!!!!!!!!!!!!.          
         !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!t################(!!!!!!!!!!!!!!!!!!!!!!!!!!!!!I$###########%(!!!!!!("j%$%3*!!!!!!!!!!!!!!!!         
       `!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!$################"!!!!!!!!!!!!!!!!!!!!!!!!!!!!$###############"!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!`       
      .!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!$##############%!!!!!!!!!!!!!!!!!!!!!!!!!!!!"################$!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.      
     `!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"$##########%*!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!$###############"!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!`     
     !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!+""""*!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!I$###########%(!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!     
    .!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"j3%%%z"*!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.    
    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!    
    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!    
    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!    
    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!(+"""+*(!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!    
    '!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!+""""""""""*!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'    
     !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!""""""""""""*!!!!"$$#################$$$3j"+!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!     
     .!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"""""""""""""!!!u############################$$J"!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.     
      '!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"""""""""""""(!!!!!((!!!!!!!!!!!!!!(*""u%$########$!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'      
       '!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!""""""""""""""*!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!+t%$%3+!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'       
        .!!!!!!!!!!!!!!!!!!!!!!!!!!!!!("""""""""""""""!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!(((*+""""""+(!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.        
          !!!!!!!!!!!!!!!!!!!!!!!!!(""""""""""""""""+!!!!!!!!!!!!!!!!!!!!(((*+"""""""""""""""""""""""!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!          
           .!!!!!!!!!!!!!!!!!!!(+"""""""""""""""""(!!!!!!!(((*+"""""""""""""""""""""""""""""""""""""(!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.           
             .!!!!!!!!!!!!!!("""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""+*(((!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.             
               .!!!!!!!!!!("""""""""""""""""""""""""""""""""""""""""""""""""+*(((!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.               
                 `'!!!!!!"""""""""""""""""""""""""""""""""""""""""""""""""""(!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'`                 
                    .!!!""""""""""""""""""""""""""""""""""""""""""""""""""""""!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.                    
                       ."""""""""""""""""""""""""""""""""""""""""""""""""""""!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'`                       
                        !""""""""""""""""""""""""""""""""""""""""""""""""""(!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.                           
                        .""""""""""""""""""""""""""""""""""""""""""""""""""""!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'.                               
                         .*""""""""""""""""""""""""""""""""""""""""""""""""""!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'.                                    
                           !""""""""""""""""""""""""""""""""""""""""""""""(!!!!!!!!!!!!!!!!!!!!!!!!!'-.                                          
                             !(""""""""""""""""""""""""""""""""""""""""""""!!!!!!!!!!!!!!!!''.``                                                 
                               `!!""""""""""""""""""""""""""""""""""""""""!..`````                                                               
                                   .!!!*"""""""""""""""""""""""""+(!!!!!.                                                                        

        _            _                  _            _       _     _         _   _          _          _             _        
       / /\         / /\               / /\         / /\    / /\  /\ \      /\_\/\_\ _     /\ \       /\ \     _    /\ \      
      / /  \       / /  \             / /  \       / / /   / / /  \ \ \    / / / / //\_\   \ \ \     /  \ \   /\_\ /  \ \     
     / / /\ \__   / / /\ \           / / /\ \__   / /_/   / / /   /\ \_\  /\ \/ \ \/ / /   /\ \_\   / /\ \ \_/ / // /\ \_\    
    / / /\ \___\ / / /\ \ \         / / /\ \___\ / /\ \__/ / /   / /\/_/ /  \____\__/ /   / /\/_/  / / /\ \___/ // / /\/_/    
    \ \ \ \/___// / /  \ \ \        \ \ \ \/___// /\ \___\/ /   / / /   / /\/________/   / / /    / / /  \/____// / / ______  
     \ \ \     / / /___/ /\ \        \ \ \     / / /\/___/ /   / / /   / / /\/_// / /   / / /    / / /    / / // / / /\_____\ 
 _    \ \ \   / / /_____/ /\ \   _    \ \ \   / / /   / / /   / / /   / / /    / / /   / / /    / / /    / / // / /  \/____ / 
/_/\__/ / /  / /_________/\ \ \ /_/\__/ / /  / / /   / / /___/ / /__ / / /    / / /___/ / /__  / / /    / / // / /_____/ / /  
\ \/___/ /  / / /_       __\ \_\\ \/___/ /  / / /   / / //\__\/_/___\\/_/    / / //\__\/_/___\/ / /    / / // / /______\/ /   
 \_____\/   \_\___\     /____/_/ \_____\/   \/_/    \/_/ \/_________/        \/_/ \/_________/\/_/     \/_/ \/___________/    
                                                                                                                              
*/
#include<bits/stdc++.h>
using namespace std;
#define ALL(obj) (obj).begin(), (obj).end()
#define REP(i, n) for(int i = 0;i < (n);++i)

#define endl '\n'
#define pb emplace_back
#define mkp make_pair
#define p_q priority_queue
#define INF 2000000000
#define PI 3.1415926

const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1};
const int dx_8[]={1,1,0,-1,-1,-1,0,1}, dy_8[]={0,-1,-1,-1,0,1,1,1};
const long long MOD = 1e9+7;

typedef long long ll;
// #define int long long

template <typename T> inline T GCD(T a,T b){T c;while(b!=0){c=a%b;a=b;b=c;}return a;}
template <typename T> inline T LCM(T a,T b){T c=GCD(a,b);a/=c;return a*b;}
template <typename T> inline T nCr(T a,T b){T i,r=1;for(i=1;i<=b;i++){r*=(a+1-i);r/=i;}return r;}
template <typename T> inline T nHr(T a,T b){return nCr(a+b-1,b);}

signed main(void){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int n; cin >> n;
  cout << n*(n-1)/2 - n/2 << endl;
  for(int i = 1; i <= n; i++){
    for(int j = i+1; j <= n; j++){
      if((n%2 == 1 && i + j != n) or (n%2 == 0 && i + j != n+1)){
        cout << i << ' ' << j << endl;
      }
    }
  }

  return 0;
}

Measured memory_bytes: 256