#include <stdio.h>

int main(){
	int i, n, s=0, znk=1;	
	
	scanf("%d", &n);
	if(n<0){
		printf("-1\n");
		return 1;
	}

	i=1;
	while(i<=n){
		s = s+znk*i;
		znk*=-1;
		i++;
	}

	printf("%d\n", s);

	return 0;
}