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
ryo_1129*/
#include<stdio.h>
//プロトタイプ宣言、引数は2つの塔の見えている高さ
int snowhight(int height_a,int height_b);

int main(void){
    //変数宣言
    int height_a,height_b,ans;
    //入力
    scanf("%d %d",&height_a,&height_b);
    //積雪量を出す関数の呼び出し
    ans = snowhight(height_a,height_b);
    //出力
    printf("%d\n",ans);

    return 0;
}
//n番目の木の高さを出す関数、等差数列の和
int snowhight(int height_a,int height_b){
    int tower_a,n,snowhight;
    //n番目とn+1番目の木の高さの違い(b-a)はn+1である。
    n = height_b-height_a-1;
    //aの高さを出す。n番目の高さは
    tower_a = (n)*(n+1)/2;
    //積雪量は（aの高さ-埋まらずに見えている高さ）
    snowhight = tower_a-height_a;
    return snowhight;
}