#include <bits/stdc++.h>
 
using namespace std;
 
ifstream fin("regine.in");
ofstream fout("regine.out");
 
int main()
{
    int n, x;
    fin >> n;
 
    x = 2*n/3+(n%3!=0);
   fout << x << '\n';
 
   x = n-x+1;
   int i;
   for(i = x; i <= n && x; i++)
   {
       fout << i << " " << x << '\n';
       x--;
   }
 
   x = i-1;
   while(i<=n)
   {
       fout << i << " " << x << '\n';
       i++;
       x--;
   }
 
    return 0;
}