Contest Duration: - (local time) (100 minutes) Back to Home
B - Stone Monument /

Time Limit: 2 sec / Memory Limit: 256 MB

### 問題文

ある村には、高さ 1,(1+2),(1+2+3),...,(1+2+3+...+999) メートルの 999 本の塔が、西から順に 1 メートル間隔で立っています。

ただし、雪は必ず 1 メートル以上積もっているものとします。

### 制約

• 1 \leq a < b < 499500(=1+2+3+...+999)
• 入力は全て整数
• 仮定が成り立たない入力は与えられない

### 入力

a b


### 入力例 1

8 13


### 出力例 1

2


2 本の塔の高さはそれぞれ 10 メートルと 15 メートルです。 よって、雪が 2 メートル積もっているとわかります。

### 入力例 2

54 65


### 出力例 2

1


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