#include <cstdio>

int line[10001];

int rec(int n){
	if(line[n] != -1) return line[n];
	if(n == 0) return line[n] = 0;
	if(n == 1) return line[n] = 2;
	return line[n] = rec(n-1) + n;
}
int main(){
	int n;
	for(int i = 0; i < 10001; i++) line[i] = -1;
	while(scanf("%d", &n) != EOF){
		printf("%d\n", rec(n));
	}
	return 0;
}