#include <cstdio>

#define Nmax 4096

#define sch(x, y) x ^= y ^= x ^= y

int v[Nmax], n, cap, coada;

int main()
{
    freopen("dame.in", "r", stdin);
    freopen("dame.out", "w", stdout);

    int i;

    scanf("%d", &n);

    if (n == 1 || n == 2)
    {
        printf("1\n");
        printf("1 1\n");
        return 0;
    }
    if (n == 3)
    {
        printf("2\n");
        printf("1 1\n3 2\n");
        return 0;
    }

    printf("%d\n", n);
    cap = 1;
    for (i = 2; i <= n; i += 2)
        v[++coada] = i;
    if (n % 12 == 3 || n %12 == 9)
    {
        for (i = 1; i < coada; ++i)
            v[i] = v[i+1];
        v[coada] = 2;
    }
    for (i = cap; i <= coada; ++i)
        printf("%d %d\n", i, v[i]);

    cap = 1; coada = 0;
    for (i = 1; i <= n; i += 2)
        v[++coada] = i;
    if (n % 12 == 8)
        for (i = 2; i <= coada; i+=2)
            sch(v[i-1], v[i]);
    if (n % 12 == 2)
    {
        sch(v[1], v[2]);
        for (i = 3; i < coada; ++i)
            v[i] = v[i+1];
        v[coada] = 5;
    }
    if (n % 12 == 3 || n%12 ==9)
    {
        for (i = 1; i <= coada-2; ++i)
            v[i] = v[i+2];
        v[coada-1] = 1;
        v[coada] = 3;
    }
    for (i = cap; i <= coada; ++i)
        printf("%d %d\n", n/2+i, v[i]);

    return 0;
}