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
  sachiyan */

#include <stdio.h>
int snow(int a, int b); //関数preのプロトタイプ宣言

int main(void){
    int west, east; //A,Bの定義
    scanf("%d %d", &west, &east); //A,Bの読み取り
    printf("%d\n", snow(west, east)); //答えを出力
}

int snow(int a, int b){ //関数preの定義　differenceを引数とする
    int pre_east; //pre_eastを定義
    int diff, answer; //差と答えの定義
    int i; //for内の変数定義
    pre_east = 0;
    diff = b - a; //高さの差を算出
    for(i = 1; i <= diff; i++){
        pre_east += i; //差と同じ値まで足し続ける
    }
    answer = pre_east - b; //関数preを呼び出し、積もる前との高さを比較する
    return(answer); //東の塔のもとの高さが分かる
}