#include<bits/stdc++.h>
using namespace std;
using ll = long long int;
using ld = long double;
#define pow(n,m) powl(n,m)
#define sqrt(n) sqrtl(n)
const ll MAX = 5000000000000000000;
const ll MOD = 1000000007;
void randinit(){srand((unsigned)time(NULL));}
int main(){
    ll N;
    cin >> N;
    N--;
    if(N % 2 == 1){
        vector<ll> A,B;
        for(ll i = 0;i < (N - 1) / 2;i++){
            for(ll j = 0;j < N - 1 - i * 2;j++){
                A.emplace_back(i);
                B.emplace_back(i + 1 + j);
            }
        }
        cout << A.size() << endl;
        for(ll i = 0;i < A.size();i++) cout << A[i] + 1 << " " << B[i] + 1 << endl;
    }
    else{
        vector<ll> A,B;
        for(ll i = 0;i < N / 2;i++){
            for(ll j = 0;j < N - 1 - i * 2;j++){
                A.emplace_back(i);
                B.emplace_back(i + 1 + j);
            }
        }
        cout << A.size() << endl;
        for(ll i = 0;i < A.size();i++) cout << A[i] + 1 << " " << B[i] + 1 << endl;
    }
}