# include <bits/stdc++.h>
using namespace std;
ifstream fi("dame.in");
ofstream fo("dame.out");
const int nmax = 1e3  +  5;
int s[nmax];
int main(void)
{
    int n;
    fi>>n;
    if (n == 2) return fo << "1\n1 1\n",0;
    if (n == 3) return fo << "2\n1 1\n3 2\n",0;
    fo << n << '\n';int p=0;
    for (int i=2;i<=n;i+=2) s[++p]=i;
    if (n % 6 == 3)
    {
        for (int i=1;i<p;++i) s[i]=s[i+1];s[p]=2;
    }
    for (int i=1;i<=p;++i) fo << i << ' ' << s[i] << '\n';
    p=0;
    for (int i=1;i<=n;i+=2) s[++p]=i;
    if (n % 12 == 8) for (int i=2;i<=p;i+=2) swap(s[i],s[i-1]);
    if (n % 12 == 2)
    {
        swap(s[1],s[2]);
        for (int i=3;i<p;++i) s[i]=s[i+1];
        s[p]=5;
    }
    if (n % 6 == 3)
    {
        for (int i=1,l=p-1;i<l;++i) s[i]=s[i+2];
        s[p-1]=1;s[p]=3;
    }
    for (int i=1,l=n/2;i<=p;++i) fo << (l + i) << ' ' << s[i] << '\n';
    return 0;
}