#include<bits/stdc++.h>
using namespace std;
const int N=110;
signed main()
{
	int n;cin>>n;
	vector<pair<int,int>>ans;
	for(int i=1;i<=n;i++)for(int j=i+1;j<=n;j++)
	{
		if(i+j==n+(n%2==0))continue;
		ans.push_back({i,j});
	}
	cout<<ans.size()<<'\n';
	for(auto i:ans)cout<<i.first<<' '<<i.second<<'\n';
	return 0;
}