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
   Sagzar1993  */

#include <stdio.h>

int calcSnowDepth(int westHeight, int eastHeight); //プロトタイプ宣言

int main(void) {
    int westHeight, eastHeight; //２つの塔の埋もれていない部分の高さ
    int snowDepth; //求める雪の深さ

    scanf("%d %d", &westHeight, &eastHeight); //入力を受け取る

    snowDepth = calcSnowDepth(westHeight, eastHeight); //積雪の深さを計算

    printf("%d\n", snowDepth); //出力

    return 0;
}

int calcSnowDepth(int westHeight, int eastHeight){
    /*
    ２つの塔が西から数えてn, n+1(=eastTowerNum)番目とすると
    eastHeight - westHeight = (n+1)(n+2)/2 - n(n+1)/2 = n+1
    だから東側の塔の番号は埋もれていない部分の高さの差に等しい
    */
    int eastTowerNum = eastHeight - westHeight;

    //１からeastTowerNumまでの和が東側の塔の高さ
    int eastTowerHeight = eastTowerNum * (eastTowerNum + 1) / 2;

    //塔の高さからその雪に埋もれていない部分の高さを引き、積雪の深さを求める
    int snowDepth = eastTowerHeight - eastHeight;

    return(snowDepth); //積雪の深さを返す
}