#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint1000000007;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001

int main(){
	
	int N;
	cin>>N;
	
	priority_queue<pair<int,int>> Q;
	Q.emplace(1,0);
	Q.emplace(1,1);
	vector<int> a(1,0),b(1,1);
	vector<int> d(N,0);
	d[0] = 1;
	d[1] = 1;
	
	for(int i=2;i<N;i++){
		vector<int> inds;
		while(true){
			pair<int,int> p = Q.top();
			Q.pop();
			if(Q.size()==0 || d[i]+1>Q.top().first+1){
				Q.push(p);
				break;
			}
			inds.push_back(p.second);
			d[i]++;
		}
		rep(j,inds.size()){
			a.push_back(i);
			b.push_back(inds[j]);
			d[inds[j]]++;
			Q.emplace(d[inds[j]],inds[j]);
		}
		Q.emplace(d[i],i);
	}
	cout<<a.size()<<endl;
	rep(i,a.size()){
		printf("%d %d\n",a[i]+1,b[i]+1);
	}
	
	return 0;
}