#include <iostream>
using namespace std;

#define MAX_N 10001

int main()
{
	int seq[MAX_N];
	int a=1;
	for(int i=0;i<MAX_N;i++){
		seq[i] = a;
		a += i + 1;
	}
	
	int n;
	while(cin>>n){
		cout<<seq[n]<<endl;
	}
	return 0;
}