Problem:
Score : 200 points

Problem Statement

In some village, there are 999 towers that are 1,(1+2),(1+2+3),...,(1+2+3+...+999) meters high from west to east, at intervals of 1 meter.

It had been snowing for a while before it finally stopped. For some two adjacent towers located 1 meter apart, we measured the lengths of the parts of those towers that are not covered with snow, and the results are a meters for the west tower, and b meters for the east tower.

Assuming that the depth of snow cover and the altitude are the same everywhere in the village, find the amount of the snow cover.

Assume also that the depth of the snow cover is always at least 1 meter.

Constraints

1 \leq a < b < 499500(=1+2+3+...+999)

All values in input are integers.

There is no input that contradicts the assumption.

Input

Input is given from Standard Input in the following format:

a b

Output

If the depth of the snow cover is x meters, print x as an integer.

Sample Input 1

8 13

Sample Output 1

2

The heights of the two towers are 10 meters and 15 meters, respectively.
Thus, we can see that the depth of the snow cover is 2 meters.

Sample Input 2

54 65

Sample Output 2

1

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Linq;
using System.Collections.Generic;
using static program.Input;
using static program.MyMath;
using static System.Console;

namespace program {
  class Program {
    static string Alp = "abcdefghijklmnopqrstuvwxyz";
    static void Main()
    {
      var a = ScanIntArray();
      var diff = a[1] - a[0];
      var s = diff * (diff+1) / 2;
      WriteLine(s-a[1]);
    }
  }
  static class Input {
    static public int ScanInt() { return int.Parse(Console.ReadLine()); }
    static public uint ScanUint() { return uint.Parse(Console.ReadLine()); }
    static public double ScanDouble() { return double.Parse(Console.ReadLine()); }
    static public long ScanLong() { return long.Parse(Console.ReadLine()); }
    static public ulong ScanUlong() { return ulong.Parse(Console.ReadLine()); }
    static public int[] ScanIntArray() { return Console.ReadLine().Split().Select(x => int.Parse(x)).ToArray(); }
    static public uint[] ScanUintArray() { return Console.ReadLine().Split().Select(x => uint.Parse(x)).ToArray(); }
    static public double[] ScanDoubleArray() { return Console.ReadLine().Split().Select(x => double.Parse(x)).ToArray(); }
    static public long[] ScanLongArray() { return Console.ReadLine().Split().Select(x => long.Parse(x)).ToArray(); }
    static public ulong[] ScanUlongArray() { return Console.ReadLine().Split().Select(x => ulong.Parse(x)).ToArray(); }

    static public string ScanStr() { return Console.ReadLine(); }
    static public string[] ScanStrArray() { return ScanStr().Split(); }
  }
  
  static class MyMath {
    public static int GCD(int a, int b) {
        Func<int, int, int> gcd = null;
        gcd = (x, y) => y == 0 ? x : gcd(y, x % y);
        return a > b ? gcd(a, b) : gcd(b, a);
    }
    public static long GCD(long a, long b) {
        Func<long, long, long> gcd = null;
        gcd = (x, y) => y == 0 ? x : gcd(y, x % y);
        return a > b ? gcd(a, b) : gcd(b, a);
    }
    public static ulong extGCD(ulong a, ulong b, ref ulong x, ref ulong y) {
      if(b==0) {
        x=1; 
        y=0;
        return a;
      }
      var d = extGCD(b, a%b, ref y, ref x);
      y -= a/b *x;
      return d;
    }
    public static int LCM(int a, int b) { return a * b / GCD(a, b); }
    public static long LCM(long a, long b) { return a * b / GCD(a, b); }
  }
}