#include<iostream>
#include<vector>
using namespace std;
int N;
main()
{
	cin>>N;
	vector<int>cnt(N,0);
	vector<pair<int,int> >E;
	for(int i=0;i<N;i++)
	{
		int t=N-i-1;
		if(cnt[i]>t)break;
		t-=cnt[i];
		for(int j=0;j<t;j++)
		{
			E.push_back(make_pair(i,i+j+1));
			cnt[i]++;
			cnt[i+j+1]++;
		}
	}
	cout<<E.size()<<endl;
	for(pair<int,int>e:E)cout<<e.first+1<<" "<<e.second+1<<endl;
}