//0057

//0{Å1,1{Å2A2{Å4A3{Å7EEE
//·Èí¿A1+sum{1=>n}(n);?
//Æè ¦¸A1`nÌñÌaÍn(n+1)/2¾
#include<stdio.h>
int main(void)
{
	int n;
	while(scanf("%d", &n) == 1)
	{
		printf("%d\n", 1+ n*(n+1)/2);
	}
	return 0;
}