数学专项counting：UVa 10079

#include <iostream>
#include <fstream>
#include <cstring>
#include <cstdio>
using namespace std;
typedef long long LL;
LL n;
int main()
{
freopen("in.txt","r",stdin);
while(cin>>n)
{
if(n<0) break;
cout<<(n*n+n)/2+1<<endl;
}
return 0;
}