import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long n = sc.nextLong();
        long k = sc.nextLong();
        long maxValue = (2*n-k+1) * k / 2 ;
        long ans = maxValue + 1;
        System.out.println(ans);
    }
}