#include<stdio.h>

#define MAXN 1001

using namespace std;

int A, B , N , i , j, v[MAXN];

int main()
{
    freopen("dame.in","r",stdin);
    freopen("dame.out","w",stdout);
    
    scanf("%d",&N);
    
    printf("%d\n",N);
    
    if( N % 2) { A = N ; B = N -1;}
     else { A = N - 1 ; B = N ;}
    
        for( i = A; i >= 1 ; i -= 2)
         v[++j] = i;
        for( i = B ; i >= 1 ; i -= 2)
         v[++j] = i;
    
 for( i = 1; i <= N ; i++)
 printf("%d %d\n",i,v[i]);
 
 
 return 0;
}