我劝出题人不要太气盛!!
傻逼题UKE
#include<bits/stdc++.h>
using namespace std;
inline int read()
{
int x=0;
bool w=0;
char c=getchar();
while(!isdigit(c)) w|=c=='-',c=getchar();
while(isdigit(c)) x=(x*10)+c-48,c=getchar();
return w?-x:x;
}
int main()
{
// freopen(".in","r",stdin);
// freopen(".out","w",stdout);
int n=read();
if(n<=8) cout<<-1;
else
{
cout<<n<<endl;
printf("1 2\n2 3\n1 4\n4 5\n5 6\n6 7\n");
for(register int i=8;i<=n;++i)
cout<<1<<' '<<i<<endl;
}
return 0;
}