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 (GCC 5.4.1) (judge verdict: Accepted)

/* ex4_3 KitamoriFumiya */
#include <stdio.h>
 
// 積雪の高さを計算して出力する関数
void snow(int west, int east){
	int difference, snow_height; // difference(西の塔の高さと東の塔の高さの差), snow_height(積雪の高さ)をint型の整数として宣言
	difference = east - west; // differenceを計算
	snow_height = 0; // 積雪の高さは初めは0としておく
	// まず、西の塔の高さを求める
	int level = 1; // 塔の高さは西から1,(1+2),(1+2+3)となっていくので初めのlevel(高さの増加分)は1とする
	// levelがdifferenceより小さい時、積雪の高さをを増加させる
	// ここで求めた積雪の高さは上の方が見えている西の塔の高さ
	while (level < difference){
		snow_height = snow_height + level; // levelの分だけ高さを増加
		level++; // 高さの増加分を1増やす
	}
	snow_height = snow_height - west; // 求めた西の塔の高さから見えている部分を引いて積雪の高さを求める
	printf("%d\n", snow_height); // 積雪の高さを出力
}
 
int main(void){
	int west, east; // west(西の塔の見えている部分の高さ), east(東の塔の見えている部分の高さ)をint型の整数として宣言
	scanf("%d %d", &west, &east); // westとeastを入力
	
	snow(west, east); // 積雪の高さを計算して出力する関数を実行
	
	return 0;
}