#include <stdio.h>
int main() {
	int n, i;
	FILE *fi = freopen("dame.in", "r", stdin);
	FILE *fo = freopen("dame.out", "w", stdout);
	scanf("%d", &n);
	if (n == 1 || n == 2)
		printf("1\n1 1\n");
	else if (n == 3)
		printf("2\n1 1\n2 3\n");
	else {
		printf("%d\n", n);
		if (n%2)
			printf("%d %d\n", n, n--);
		for (i = 1; i <= n/2; ++i)
			printf("%d %d\n%d %d\n", i, 2*i, n+1-i, n+1-2*i);
	}
	return 0;
}