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
	ruuu0048
*/
#include <stdio.h>

int sigma(int max_num); //1から引数までの数の和を返す関数
int answer(int west_height,int east_height); //答えである雪の深さを返す関数

int main(void){

    int west_height,east_height; //西と東の塔の見えるところの高さの変数を宣言

    scanf("%d %d",&west_height,&east_height); //それぞれ入力

    printf("%d\n",answer(west_height,east_height));//答えを出力

    return 0; 
}

int sigma(int max_num){
    int total = 0; //足しあわせていく変数を宣言

    while(max_num > 0){ //max_numが0になったらループが終わる
        total += max_num; //足していく
        max_num -= 1; //足したら1減らす
    }
    return total;

};

int answer(int west_height,int east_height){
    
    int real_east_height,snow_height; //東側の塔全体の高さと積雪の高さの変数を宣言

    real_east_height = sigma(east_height - west_height); //東側の塔全体の高さを計算

    snow_height = real_east_height - east_height; //積雪の高さを計算

    return snow_height;

};