UVA 10079 Pizza Cutting

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <cstdio>
#include <cmath>
using namespace std;

typedef long long LL;

LL n;

int main()
{
	while(~scanf("%lld", &n))
	{
		if(n < 0) break;
		printf("%lld\n", (n*n+n)/2+1);
	}
	return 0;
}