import java.util.*;
import java.io.*;

public class Main {
	public static void main(String[] args) throws Exception {
		new Main().run();
	}

	void run() throws Exception {
		Scanner sc = new Scanner(System.in);
		long N = sc.nextLong();
		long K = sc.nextLong();
		long ans = 1;
		for (int i = 0; i < K; ++i) {
			ans += N - i;
		}
		System.out.println(ans);
	}

	void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}

}