#include<fstream>

using namespace std;
ifstream F("dame.in");
ofstream G("dame.out");
int n, i, j;

int main() {
    F >> n;
    if (n == 1 || n == 2)
        G << "1\n1 1";
    else if (n == 3)
        G << "2\n1 1\n2 3";
    else {
        G << n << '\n';
        if (n % 2)
            G << n << ' ' << n << '\n', --n;
        if (n % 6 == 2) {
            for (i = n / 2, j = 1; i <= n; i += 2, ++j)
                G << i << ' ' << j << '\n' << n + 1 - i << ' ' << n + 1 - j << '\n';
            for (i = 2 - i % 2; j <= n / 2; i += 2, ++j)
                G << i << ' ' << j << '\n' << n + 1 - i << ' ' << n + 1 - j << '\n';
        } else
            for (i = 1; i <= n / 2; ++i)
                G << i << ' ' << 2 * i << '\n' << n + 1 - i << ' ' << n + 1 - 2 * i << '\n';
    }
    return 0;
}