#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>


int main(int argc, char *argv[])
{
	int L = 0;
	scanf("%d", &L);

	long long int  O = 0;
	for (int i = 1; i <= L - 2; i += 2)
	{
		O += i;
	}

	printf("%lld", O);

	return 0;

}