#include <iostream>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */

int main( )
{
	int i,sum=0;
	int n;
	scanf("%d",i);
	for(int j=0;j<=i;j++)
	{
		if(j=2*n)
			j=j*(-1);
		else
		    j=j*1;
		sum+=j;
	}
	printf("%d \n",sum);
	getchar();
	return 0;
}