#include <cstdio>
using namespace std;

int sygma(int n){
	if(n == 1) return 1;
	
	return n + sygma(n - 1);
}

int main(){
	int n;
	while(scanf("%d", &n) != EOF){
		printf("%d\n", 1 + sygma(n));
	}
	
	return 0;
}