import java.util.*; import java.io.*; import java.math.*; public class Main{ static void solve(){//Here is the main function ArrayList one = nextLongArray(); long N = one.get(0); long K = one.get(1); myout((2 * N - K + 1)* K / 2); } //Method addition frame start //Method addition frame end //Don't have to see. start------------------------------------------ static class InputIterator{ ArrayList inputLine = new ArrayList<>(1024); int index = 0; int max; String read; InputIterator(){ BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); try{ while((read = br.readLine()) != null){ inputLine.add(read); } }catch(IOException e){} max = inputLine.size(); } boolean hasNext(){return (index < max);} String next(){ if(hasNext()){ return inputLine.get(index++); }else{ throw new IndexOutOfBoundsException("There is no more input"); } } } static HashMap CONVSTR = new HashMap<>(); static InputIterator ii = new InputIterator();//This class cannot be used in reactive problem. static PrintWriter out = new PrintWriter(System.out); static void flush(){out.flush();} static void myout(Object t){out.println(t);} static void myerr(Object t){System.err.print("debug:");System.err.println(t);} static String next(){return ii.next();} static boolean hasNext(){return ii.hasNext();} static int nextInt(){return Integer.parseInt(next());} static long nextLong(){return Long.parseLong(next());} static double nextDouble(){return Double.parseDouble(next());} static ArrayList nextStrArray(){return myconv(next(), 8);} static ArrayList nextCharArray(){return myconv(next(), 0);} static ArrayList nextIntArray(){ ArrayList input = nextStrArray(); ArrayList ret = new ArrayList<>(input.size()); for(int i = 0; i < input.size(); i++){ ret.add(Integer.parseInt(input.get(i))); } return ret; } static ArrayList nextLongArray(){ ArrayList input = nextStrArray(); ArrayList ret = new ArrayList<>(input.size()); for(int i = 0; i < input.size(); i++){ ret.add(Long.parseLong(input.get(i))); } return ret; } @SuppressWarnings("unchecked") static String myconv(Object list, int no){//only join String joinString = CONVSTR.get(no); if(list instanceof String[]){ return String.join(joinString, (String[])list); }else if(list instanceof ArrayList){ return String.join(joinString, (ArrayList)list); }else{ throw new ClassCastException("Don't join"); } } static ArrayList myconv(String str, int no){//only split String splitString = CONVSTR.get(no); return new ArrayList(Arrays.asList(str.split(splitString))); } public static void main(String[] args){ CONVSTR.put(8, " "); CONVSTR.put(9, "\n"); CONVSTR.put(0, ""); solve();flush(); } //Don't have to see. end------------------------------------------ }