using System;
using System.Linq;
using System.Collections.Generic;

namespace Contest
{
  class Program
  {
    static void Main(string[] args)
    {
      ABC099.B();
    }
    class ABC099
    {
      public static void A()
      {
        var N = Read.Int();
        Console.WriteLine(N <= 999 ? "ABC" : "ABD");
      }
      public static void B()
      {
        var A = Read.Int();
        var B = Read.Int();
        var d = B - A;
        Console.WriteLine(d * (d - 1) / 2 - A);
      }
      public static void C()
      {
        var N = Read.Int();
        var min = N;
        var P6 = 46656;
        var P9 = 59049;
        for (int i = 0; i <= N; ++i)
        {
          var count = 0;
          var v = i;
          var p = P6;
          while (v > 0)
          {
            while (p > v)
            {
              p /= 6;
            }
            v -= p;
            count++;
          }
          v = N - i;
          p = P9;
          while (v > 0)
          {
            while (p > v)
            {
              p /= 9;
            }
            v -= p;
            count++;
          }
          min = Math.Min(min, count);
        }
        Console.WriteLine(min);
      }
      public static void D()
      {
        var N = Read.Int();
        var C = Read.Int();
        var D = Enumerable.Range(0, C).Select(i => Read.IntN()).ToArray();
        var G = Enumerable.Range(0, N).Select(i => Read.IntN()).ToArray();
        var board = new int[3, C];
        for(int i = 0; i < N; ++i)
        {
          for(int j = 0; j < N; ++j)
          {
            board[(i + j) % 3, G[i][j] - 1]++;
          }
        }
        var bad = new int[3, C];
        for(int i = 0; i < 3; ++i)
        {
          for(int j = 0; j < C; ++j)
          {
            for(int k = 0; k < C; ++k)
            {
              bad[i, j] += D[k][j] * board[i, k];
            }
          }
        }
        var min = Int32.MaxValue;
        for(int i = 0; i < C; ++i)
        {
          var b1 = bad[0, i];
          for(int j = 0; j < C; ++j)
          {
            if (i == j) { continue; }
            var b2 = bad[1, j];
            for(int k = 0; k < C; ++k)
            {
              if (i == k || j == k) { continue; }
              var b3 = bad[2, k];
              min = Math.Min(min, b1 + b2 + b3);
            }
          }
        }
        Console.WriteLine(min);
      }
    }

    class Util
    {
      public static T[,] Gen<T>(int r, int c, T v)
      {
        var matrix = new T[r, c];
        for (int i = 0; i < r; ++i)
        {
          for (int j = 0; j < c; ++j)
          {
            matrix[i, j] = v;
          }
        }
        return matrix;
      }
      public static long[] ASum(long[] array)
      {
        var res = new long[array.Length + 1];
        for (int i = 0; i < array.Length; ++i)
        {
          res[i + 1] = res[i] + array[i];
        }
        return res;
      }
      public static long[,] ASum(long[,] matrix)
      {
        var N = matrix.GetLength(0);
        var M = matrix.GetLength(1);
        var res = new long[N + 1, M + 1];
        for(int i = 0; i < N; ++i)
        {
          for(int j = 0; j < M; ++j)
          {
            res[i + 1, j + 1] = res[i, j + 1] + res[i + 1, j] - res[i, j] + matrix[i, j];
          }
        }
        return res;
      }
      public static Dictionary<T, long> Frequency<T>(T[] array)
      {
        var dict = new Dictionary<T, long>();
        foreach (var a in array)
        {
          if (!dict.ContainsKey(a)) { dict.Add(a, 0); }
          dict[a]++;
        }
        return dict;
      }
    }

    static class Read
{
static Queue<string> queue = new Queue<string>();
static void Enqueue() { var ss = Console.ReadLine().Split(' '); foreach (var s in ss) { queue.Enqueue(s); } }
public static int Int() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt32(queue.Dequeue()); }
public static long Long() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt64(queue.Dequeue()); }
public static string Str() { if (queue.Count == 0) { Enqueue(); } return queue.Dequeue(); }
public static int[] IntN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt32(s)).ToArray(); }
public static long[] LongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt64(s)).ToArray(); }
public static ulong[] ULongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToUInt64(s)).ToArray(); }
public static string[] StrN() { return Console.ReadLine().Split(' '); }
public static Tuple<int, int>[] TupleInt(int n) { List<Tuple<int, int>> list = new List<Tuple<int, int>>(); for (int i = 0; i < n; ++i) { var ab = Read.IntN(); list.Add(new Tuple<int, int>(ab[0], ab[1])); } return list.ToArray(); }
public static Tuple<long, long>[] TupleLong(int n) { List<Tuple<long, long>> list = new List<Tuple<long, long>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<long, long>(ab[0], ab[1])); } return list.ToArray(); }
public static Tuple<string, string>[] TupleStr(int n) { List<Tuple<string, string>> list = new List<Tuple<string, string>>(); for (int i = 0; i < n; ++i) { var ab = Read.StrN(); list.Add(new Tuple<string, string>(ab[0], ab[1])); } return list.ToArray(); }
public static Tuple<double, double>[] TupleDouble(int n) { List<Tuple<double, double>> list = new List<Tuple<double, double>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<double, double>(ab[0], ab[1])); } return list.ToArray(); }
public static long[,] LongMatrix(int r, int c) { var mat = new long[r, c]; for (int i = 0; i < r; ++i) { var x = Read.LongN(); for (int j = 0; j < c; ++j) { mat[i, j] = x[j]; } } return mat; }
public static Dictionary<int, HashSet<int>> AdjacencyList(int N, int M, bool isDirectional = false)
{
Dictionary<int, HashSet<int>> dict = new Dictionary<int, HashSet<int>>();
for (int i = 0; i <= N; ++i) { dict.Add(i, new HashSet<int>()); }
        for (int i = 0; i < M; ++i) { var ab = Read.IntN(); if (!dict[ab[0]].Contains(ab[1])) { dict[ab[0]].Add(ab[1]); } if (!isDirectional && !dict[ab[1]].Contains(ab[0])) { dict[ab[1]].Add(ab[0]); } }
        return dict;
}
public static Dictionary<int, Dictionary<int, long>> AdjacencyListWithWeight(int N, int M)
{
Dictionary<int, Dictionary<int, long>> dict = new Dictionary<int, Dictionary<int, long>>(); for (int i = 0; i <= N; ++i) { dict.Add(i, new Dictionary<int, long>()); }
for (int i = 0; i < M; ++i) { var ab = Read.IntN(); dict[ab[0]].Add(ab[1], ab[2]); dict[ab[1]].Add(ab[0], ab[2]); }
return dict;
}
}
    }
}


Measured memory_bytes: 13396