#include<fstream>

using namespace std;

ifstream cin("dame.in");
ofstream cout("dame.out");

void dame(int n){
cout<<n<<'\n';
    for(int i=1;i<=(n+n%2);i++){
        if(i%2==1){cout<<i<<' '<<(i/2+1)<<'\n';}else{if(n%2==0){cout<<i<<' '<<i/2+n/2<<'\n';}else{if(i<n+n%2){cout<<i<<' '<<i/2+n/2+1<<'\n';}}}
    }
}
main(){

    int n;
    cin>>n;
    dame(n);
}