#include <iostream>

using namespace std;

int main()
{
    int n,v,sum = 0,money=0;
    cin >> n >> v;

    if(n>v)
    {

            money = v;

        int counnt = n - (v-1);

        for(int i=2; i<counnt; i++)
        {
            money = money + i;
        }
        cout << money << endl;
        return 0;
    }
    else if( v>=n)
       cout<<n-1<<endl;
       return 0;
    }
}