#include<cstdio>
#include<iostream>

using namespace std;

int main()
{
    int a,n,sum=0;
    cin>>a;
    while(cin>>n,n<=0);//逗号表达式取逗号后的值
    for(int i=0;i<n;i++)
    {
        sum+=a+i;
    }
    cout<<sum;
}