import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int k = sc.nextInt();
		sc.close();

		long ans = 1;
		for (int i = 0; i < k; i++) {
			ans += n - i;
		}
		System.out.println(ans);
	}
}