Codeforces 40-E

#include <iostream>
using namespace std;

int main()
{
        int n;
        cin>>n;
        if(n%2==0)
                cout<<n/2*n/2<<endl;
        else
                cout<<n/2*(n+1)/2<<endl;
        for(int i=1;i<=n/2;i++)
                for(int j=n/2+1;j<=n;j++)
                        cout<<i<<" "<<j<<endl;
        return 0;
}