#include <cstdio>

int line[10001];

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