#include <bits/stdc++.h>
using namespace std;
int n ;
int notPrime[2005];
void sieve(){
    for(int i=4;i<=2000;i+=2)notPrime[i]=1;
    for(int i=3;i<=2000;i+=2){
        if(notPrime[i])continue;
        for(int j=i+i;j<=2000;j+=i){
            notPrime[j]=1;
        }
    }
}
int adj[1003][1003];
vector<pair<int,int> > edge;
int deg[1003];
void moreEdge(){
    for(int i=1;i<=n;i++){
        if(deg[i]==3)continue;
        assert(deg[i]==2);
        for(int j=i+1;j<=n;j++){
            if(deg[j]==3)continue;
            assert(deg[j]==2);
            if(adj[i][j]||adj[j][i])continue;
            edge.push_back({i,j});
            deg[i]++;
            deg[j]++;
            break;
        }
        if(notPrime[edge.size()]==0)return ;
    }
    assert(false);
}
int main(){
    sieve();
    cin>>n;
    for(int i=1;i<n;i++){
        adj[i][i+1]=1;
        adj[i+1][i]=1;
        deg[i]=2;
        edge.push_back({i,i+1});
    }

    adj[n][1]=1;
    adj[1][n]=1;
    deg[n]=2;
    edge.push_back({n,1});


    if(notPrime[edge.size()])moreEdge();

    cout << edge.size() << endl;
    for(auto e:edge){
        printf("%d %d\n",e.first,e.second);
    }


    return 0;

}