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
        Tarara  */
#include <stdio.h>

int cal_snow_height(int west_residual_height, int east_residual_height);
int cal_tower_height(int i);

int main(void)
{
    int west_residual_height; // 西側の埋もれていない高さ
    int east_residual_height; // 東側の埋もれていない高さ
    scanf("%d %d", &west_residual_height, &east_residual_height);

    printf("%d\n", cal_snow_height(west_residual_height, east_residual_height));
    return 0;
}

// 積雪量を計算する
int cal_snow_height(int west_residual_height, int east_residual_height)
{
    // 東側の塔の番号
    int east_ind = east_residual_height - west_residual_height;

    // 東側の塔の高さ - 東側の塔の埋もれていない部分 = 降雪量
    return cal_tower_height(east_ind) - east_residual_height;
}

// 西からi番目の塔の高さを求める
int cal_tower_height(int i)
{
    return i * (i + 1) / 2;
}