#include<iostream>
#include<bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define rep_r(i, n) for(int i = n-1; i >= 0; i--)
#define rep1(i, n) for(int i = 1; i <= (int)(n); i++)
#define REP(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define all(x) (x).begin(),(x).end()
#define SZ(x) ((ll)(x).size())
#define bit(n) (1LL<<(n))
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

#define INF bit(60)
#define pb push_back
#define mod 1000000007

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

using namespace std;
using uif=uint_fast64_t;
using ll=long long int;
using tTree=__gnu_pbds::tree< ll , __gnu_pbds::null_type, less< ll >, __gnu_pbds::rb_tree_tag, __gnu_pbds::tree_order_statistics_node_update>;

int qp(int a,ll b){int ans=1;do{if(b&1)ans=1ll*ans*a%mod;a=1ll*a*a%mod;}while(b>>=1);return ans;}
int qp(int a,ll b,int mo){int ans=1;do{if(b&1)ans=1ll*ans*a%mo;a=1ll*a*a%mo;}while(b>>=1);return ans;}

int gcd(int a,int b){return b?gcd(b,a%b):a;}
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};


#define FACSIZE 2002

ll invfac[FACSIZE];
ll fac[FACSIZE];

ll mod_pow(ll ,ll );
ll mod_add(ll ,ll );
ll mod_mul(ll ,ll );
ll mod_div(ll ,ll );
void make_fact(ll );
void make_invfact(ll );



int main(void){

  ll n;
  cin>>n;
  using pa=pair<ll,ll>;

  if( n%2==0 ){

    vector<pa>  vec;
    ll sum=n+1;
    for(ll i=1;i<=n/2;i++){
      vec.push_back( make_pair(i,sum-i) );
    }
    //cout << vec.size()*vec.size() << endl;
    cout << n*(n-2)/2 << endl;
    while(!vec.empty()){

      auto p=vec.back();
      vec.pop_back();
      for(auto pp:vec){
        cout << p.first << " " << pp.first << endl;
        cout << p.first << " " << pp.second<< endl;
        cout << p.second << " " << pp.first<< endl;
        cout << p.second << " " << pp.second<< endl;
      }

    }

    return 0;
  }
  else{
    vector<pa>  vec;
    ll sum=n;
    for(ll i=1;i<=n/2;i++){
      vec.push_back( make_pair(i,sum-i) );
    }
    //cout << vec.size()*(vec.size()+1) << endl;
    cout << (n-1)*(n-3)/2 + 2*vec.size() << endl;
    for(auto pp:vec){
      cout << sum << " "<< pp.first << endl;
      cout << sum << " "<< pp.second << endl;
    }

    while(!vec.empty()){

      auto p=vec.back();
      vec.pop_back();
      for(auto pp:vec){
        cout << p.first << " " << pp.first << endl;
        cout << p.first << " " << pp.second<< endl;
        cout << p.second << " " << pp.first<< endl;
        cout << p.second << " " << pp.second<< endl;
      }

    }

  }

  return 0;

}

ll mod_pow(ll x,ll r){
  if(r==0) return 1;
  else if(r==1) return x%mod;
  else if(r%2==0){
    ll t=mod_pow(x,r/2)%mod;
    return t*t %mod;
  } 
  else{
    ll t=mod_pow(x,r/2)%mod;
    ll k= t*t %mod;
    return k*x%mod;
  } 
}
ll mod_add(ll a,ll b){
  return (a+b)%mod;
}

ll mod_mul(ll a,ll b){
  return (a*b)%mod;
}

ll mod_div(ll a,ll b){
  return mod_mul(a,mod_pow(b,mod-2));
}

void make_fact(ll n){
  fac[0]=1;
  rep(i,n){
    fac[i+1]=mod_mul(fac[i],i+1);
  }  
}

void make_invfact(ll n){
  invfac[n]= mod_pow( fac[n] ,mod-2);
  for(int i=n-1;i>=0;i--){
    invfac[i]=mod_mul(invfac[i+1],i+1);
  }

}

Measured memory_bytes: 256