1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
#include <iostream>

using namespace std;

int main()
{
    int n;
    cin>>n;

    if(n==1)
    {
        cout<<"2\n2 -1\n-1 -1\n";
        return 0;
    }

    cout<<n+1<<endl;
    for(int i=0; i<n+1; i++)
    {
        if(n%2==0)
        {
            if(i%2==0 && i!=n)
                cout<<i+2<<" "<<i+3<<endl;
            else if(i%2==1)
                cout<<i+2<<" -1"<<endl;
            else cout<<"-1 -1"<<endl;
        }
        else
        {
            if(i%2==1 && i!=n)
                cout<<i+2<<" "<<i+3<<endl;
            else if(i==0)
                cout<<i+2<<" "<<n+1<<endl;
            else if(i%2==0)
                cout<<i+2<<" -1"<<endl;
            else cout<<"-1 -1"<<endl;
        }
    }

    return 0;
}