using System; using System.Collections.Generic; using System.Linq; using System.IO; using System.Text; using System.Numerics; using System.Threading; using System.Runtime.CompilerServices; using System.Diagnostics; using static System.Math; using static System.Array; using static AtCoder.Sc_out; using static AtCoder.Tool; using static AtCoder.ModInt; namespace AtCoder { class AC { const int MOD = 1000000007; //const int MOD = 998244353; const int INF = int.MaxValue / 2; const long SINF = long.MaxValue / 2; const double EPS = 1e-8; static readonly int[] dI = { 0, 1, 0, -1, 1, -1, -1, 1 }; static readonly int[] dJ = { 1, 0, -1, 0, 1, 1, -1, -1 }; //static List> G = new List>(); //static List> G = new List>(); //static List E = new List(); static void Main(string[] args) { //var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false }; Console.SetOut(sw); var th = new Thread(Run, 1 << 26); th.Start(); th.Join(); //Run(); Console.Out.Flush(); } static void Run() { int Testcase = 1; for (var _ = 0; _ < Testcase; _++) Solve(); } static void Solve() { Cin.Input(out long N, out long K); long MX = N * (N + 1) / 2; long MN = N * (N + 1) / 2 - K * (2 * N - K + 1); OutL((MX - MN) / 2 + 1); } public struct Edge { public int from; public int to; public long dist; public Edge(int t, long c) { from = -1; to = t; dist = c; } public Edge(int f, int t, long c) { from = f; to = t; dist = c; } } } public class Priority_Queue { private List Q; private readonly Comparison Func_Compare; public Priority_Queue(Comparison comp) { Func_Compare = comp; Q = new List(); } private void PushHeap(List list, T item) { int n = list.Count(); list.Add(item); while (n != 0) { int pIndex = (n - 1) / 2; if (Func_Compare(list[n], list[pIndex]) < 0) { Swap(Q, n, pIndex); } else { break; } n = pIndex; } } private void PopHeap(List list) { int n = list.Count() - 1; list[0] = list[n]; list.RemoveAt(n); int cur = 0; int comp; while (2 * cur + 1 <= n - 1) { int c1 = 2 * cur + 1; int c2 = 2 * (cur + 1); if (c1 == n - 1) { comp = c1; } else { comp = Func_Compare(list[c1], list[c2]) < 0 ? c1 : c2; } if (Func_Compare(list[cur], list[comp]) > 0) { Swap(Q, cur, comp); } else { break; } cur = comp; } } private void Swap(List list, int a, int b) { T keep = list[a]; list[a] = list[b]; list[b] = keep; } public void Enqueue(T value) { PushHeap(Q, value); } public T Dequeue() { T ret = Q[0]; PopHeap(Q); return ret; } public T Peek() { return Q[0]; } public int Count() { return Q.Count(); } public bool Any() { return Q.Any(); } } public class SegmentTree { //1-indexed type int n; T[] Tree; Func f; T ex; int L; [MethodImpl(MethodImplOptions.AggressiveInlining)] public SegmentTree(int size, Func fun, T exvalue) { ex = exvalue; f = fun; n = 1; while (n < size) n <<= 1; Tree = new T[n << 1]; L = (n << 1) - 1; for (var i = 0; i <= L; i++) Tree[i] = ex; } [MethodImpl(MethodImplOptions.AggressiveInlining)] public SegmentTree(int size, Func fun, T exvalue, T[] initial) { ex = exvalue; n = 1; while (n < size) n <<= 1; f = fun; Tree = new T[n << 1]; L = (n << 1) - 1; for (var i = 0; i <= L; i++) Tree[i] = (n <= i && i <= n + initial.Length - 1) ? initial[i - n] : ex; } [MethodImpl(MethodImplOptions.AggressiveInlining)] public void Set_All() { for (var i = n - 1; i >= 1; i--) Tree[i] = f(Tree[i << 1], Tree[(i << 1) | 1]); } [MethodImpl(MethodImplOptions.AggressiveInlining)] public void Assign(int idx, T nxt) => Tree[idx + n] = nxt; [MethodImpl(MethodImplOptions.AggressiveInlining)] public void Update(int idx) { int now = idx + n; while (now > 1) { now >>= 1; Tree[now] = f(Tree[now << 1], Tree[now << 1 | 1]); } } [MethodImpl(MethodImplOptions.AggressiveInlining)] public void Query_Update(int idx, T nxt) { Assign(idx, nxt); Update(idx); } [MethodImpl(MethodImplOptions.AggressiveInlining)] public void Query_Update_func(int idx, T y) { Assign(idx, f(Peek(idx), y)); Update(idx); } [MethodImpl(MethodImplOptions.AggressiveInlining)] public T Query_Fold(int l, int r) { int L = n + l; int R = n + r; T vL = ex, vR = ex; while (L < R) { if (L % 2 == 1) { vL = f(vL, Tree[L]); L++; } if (R % 2 == 1) { vR = f(Tree[R - 1], vR); R--; } L >>= 1; R >>= 1; } return f(vL, vR); } [MethodImpl(MethodImplOptions.AggressiveInlining)] public T Peek(int idx) => Tree[idx + n]; [MethodImpl(MethodImplOptions.AggressiveInlining)] public void Display(int len) { for (var i = 0; i < len; i++) Console.Write($"{Tree[i + n]} "); Console.WriteLine(); } } public class UnionFind { private int[] parent; private int[] rank; private int[] size; public UnionFind(int n) { parent = new int[n]; rank = new int[n]; size = new int[n]; for (var i = 0; i < n; i++) { parent[i] = i; rank[i] = 0; size[i] = 1; } } public int Root(int x) { return parent[x] == x ? x : parent[x] = Root(parent[x]); } public bool SameRoot(int x, int y) { return Root(x) == Root(y); } public void Unite(int x, int y) { x = Root(x); y = Root(y); if (x == y) { return; } if (rank[x] < rank[y]) { parent[x] = y; size[y] += size[x]; size[x] = 0; } else { parent[y] = x; if (rank[x] == rank[y]) { rank[x]++; } size[x] += size[y]; size[y] = 0; } } public int SizeOf(int x) { return size[Root(x)]; } } struct ModInt { public long value; private const int MOD = 1000000007; //private const int MOD = 998244353; public ModInt(long value) { this.value = value; } [MethodImpl(MethodImplOptions.AggressiveInlining)] public static implicit operator ModInt(long a) { var ret = a % MOD; return new ModInt(ret < 0 ? (ret + MOD) : ret); } [MethodImpl(MethodImplOptions.AggressiveInlining)] public static ModInt operator +(ModInt a, ModInt b) => (a.value + b.value); [MethodImpl(MethodImplOptions.AggressiveInlining)] public static ModInt operator -(ModInt a, ModInt b) => (a.value - b.value); [MethodImpl(MethodImplOptions.AggressiveInlining)] public static ModInt operator *(ModInt a, ModInt b) => (a.value * b.value); [MethodImpl(MethodImplOptions.AggressiveInlining)] public static ModInt operator /(ModInt a, ModInt b) => a * Modpow(b, MOD - 2); public static ModInt operator <<(ModInt a, int n) => (a.value << n); public static ModInt operator >>(ModInt a, int n) => (a.value >> n); public static ModInt operator ++(ModInt a) => a.value + 1; public static ModInt operator --(ModInt a) => a.value - 1; [MethodImpl(MethodImplOptions.AggressiveInlining)] public static ModInt Modpow(ModInt a, long n) { var k = a; ModInt ret = 1; while (n > 0) { if ((n & 1) != 0) ret *= k; k *= k; n >>= 1; } return ret; } private static readonly List Factorials = new List() { 1 }; [MethodImpl(MethodImplOptions.AggressiveInlining)] public static ModInt Fac(long n) { for (var i = Factorials.Count(); i <= n; i++) { Factorials.Add((Factorials[i - 1] * i) % MOD); } return Factorials[(int)n]; } [MethodImpl(MethodImplOptions.AggressiveInlining)] public static ModInt nCr(long n, long r) { return n < r ? 0 : Fac(n) / (Fac(r) * Fac(n - r)); } public static explicit operator int(ModInt a) => (int)a.value; } static class Cin { public static string[] ReadSplit => Console.ReadLine().Split(); public static int[] ReadSplitInt => ConvertAll(Console.ReadLine().Split(), int.Parse); public static long[] ReadSplitLong => ConvertAll(Console.ReadLine().Split(), long.Parse); public static double[] ReadSplit_Double => ConvertAll(Console.ReadLine().Split(), double.Parse); public static string Str => Console.ReadLine(); public static int Int => int.Parse(Console.ReadLine()); public static long Long => long.Parse(Console.ReadLine()); public static double Double => double.Parse(Console.ReadLine()); public static T Conv(string input) { if (typeof(T).Equals(typeof(ModInt))) { return (T)(dynamic)(long.Parse(input)); } return (T)Convert.ChangeType(input, typeof(T)); } public static void Input(out T a) => a = Conv(Console.ReadLine()); public static void Input(out T a, out U b) { var q = ReadSplit; a = Conv(q[0]); b = Conv(q[1]); } public static void Input(out T a, out U b, out V c) { var q = ReadSplit; a = Conv(q[0]); b = Conv(q[1]); c = Conv(q[2]); } public static void Input(out T a, out U b, out V c, out W d) { var q = ReadSplit; a = Conv(q[0]); b = Conv(q[1]); c = Conv(q[2]); d = Conv(q[3]); } public static void Input(out T a, out U b, out V c, out W d, out X e) { var q = ReadSplit; a = Conv(q[0]); b = Conv(q[1]); c = Conv(q[2]); d = Conv(q[3]); e = Conv(q[4]); } } static class Sc_out { public static void OutL(object s) => Console.WriteLine(s); public static void Out_Sep(IEnumerable s) => Console.WriteLine(string.Join(" ", s)); public static void Out_Sep(IEnumerable s, string sep) => Console.WriteLine(string.Join($"{sep}", s)); public static void Out_Sep(params object[] s) => Console.WriteLine(string.Join(" ", s)); public static void Out_One(object s) => Console.Write($"{s} "); public static void Out_One(object s, string sep) => Console.Write($"{s}{sep}"); public static void Endl() => Console.WriteLine(); } public static class Tool { static public void Initialize(ref T[] array, T initialvalue) { array = ConvertAll(array, x => initialvalue); } static public void Swap(ref T a, ref T b) { T keep = a; a = b; b = keep; } static public void Display(T[,] array2d, int n, int m) { for (var i = 0; i < n; i++) { for (var j = 0; j < m; j++) { Console.Write($"{array2d[i, j]} "); } Console.WriteLine(); } } static public long Gcd(long a, long b) { if (a == 0 || b == 0) return Max(a, b); return a % b == 0 ? b : Gcd(b, a % b); } static public long LPow(int a, int b) => (long)Pow(a, b); static public bool Bit(long x, int dig) => ((1L << dig) & x) != 0; static public int Sig(long a) => a == 0 ? 0 : (int)(a / Abs(a)); } }