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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.Comparator;
import java.util.Scanner;
import java.util.ArrayList;
import java.util.PriorityQueue;
import java.util.Arrays;
	/* static int java.util.Arrays.binarySearch(int[] a, int key); */
import java.math.BigDecimal;
	
 
 
class Main {
	Scanner sc = new Scanner(System.in);
	int a, b;
	public void run(){
		a = sc.nextInt();
		b = sc.nextInt();
		solve();
	}
	public void solve(){
		int sabun = b - a;
		int ans = 0;
		for(int i = 0; i < sabun; i++){
			ans += i;
		}

		ans -= a;

		System.out.println(ans);
	}
	
	public static void main(String[] args){
		new Main().run();
	}
	
	static class Pair {
        int first,second;
        public Pair(int a,int b) {
            this.first = a;
            this.second = b;
        }
    }
	static class Edge{
		int to, cost;
		public Edge(int a, int b){
			this.to = a;
			this.cost = b;
		}
	}
	class MyComparator implements Comparator {
		@Override
		public int compare (Object arg0, Object arg1) {
			int x = ((Pair)arg0).first;
			int y = ((Pair)arg1).first;
			
			if (x < y) {
				return 1;
			} else if (x > y) {
				return -1;
			} else{
				return 0;
			}
		}
	}
	static class UnionFind{
		int par[];
		int rank[];
		
		//n要素で初期化
		public UnionFind(int maxn){
			par = new int[maxn];
			rank = new int[maxn];
			
			for(int i = 0; i < maxn; i++){
				par[i] = i;
				rank[i] = 0;
			}
		}
		
		//木の根を求める
		int find(int x){
			if(par[x] == x){
				return x;
			}
			else {
				return par[x] = find(par[x]);
			}
		}
		
		// xとyの属する集合を併合
		void unite (int x, int y){
			x = find(x);
			y = find(y);
			if(x == y){
				return;
			}
			
			if(rank[x] < rank[y]){
				par[x] = y;
			}
			else{
				par[y] = x;
				if(rank[x] == rank[y]) rank[x]++;
			}
		}
		
		// xとyが同じ集合に属するか否か
		boolean same(int x, int y){
			return find(x) == find(y);
		}
	}
}