#include<iostream>

using namespace std;
int main()
{
    int n,m;
    cin>>n>>m;
    int total=2*m;
    for (int i=m+1;i<=n;i++)
    total+=i;
    cout<<total;

}

/**************************************************************
	Problem: 1370
	User: admin
	Language: C++
	Result: Accepted
	Time:15 ms
	Memory:2072 kb
****************************************************************/