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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using intl = long long;
using itnl = long long;
using itn = int;
using ld = long double;
using namespace std;
#define rep(i, n) for(intl i = 0; i < (intl)(n); i++)
#define rrep(i, n) for(intl i = (intl)(n) - 1; i >= 0; i--)
#define repi(i, a, b) for(intl i = (intl)(a); i < (intl)(b); i++)
#define rrepi(i, a, b) for(intl i = (intl)(a); i > (intl)(b); i--)
#define max(a, b) ((a) > (b) ? (a) : (b))
#define min(a, b) ((a) < (b) ? (a) : (b))
#define all(x) (x).begin(),(x).end()
#define m0(x) memset(x,0,sizeof(x))
#define m1(x) memset(x,1,sizeof(x))
#define fill(x,y) memset(x,y,sizeof(x))
#define alength(a) (sizeof(a) / sizeof(a[0]))
#define cin(i, a) rep(i, alength(a))cin >> a[i]
#define ccin(i, j, a) rep(i, alength(a))rep(j, alength(a[i]))cin >> a[i][j]
#define debug(x) cout << #x << ":" << x << endl
#define rdebug(i,x) rep(i, alength(x))cout << #x << "[" << i << "]:" << x[i] << endl
#define sqrt sqrtl
#define itn int
#define double ld
#define pb push_back
#define mp make_pair
const intl INF = 1e9;
const intl MOD = 1e9+7;
const ld eps = 1.0e-14;//-9
void fp(bool f){cout << (f ? "Yes" : "No") << endl;}
void fp(bool f, string s, string t){cout << (f ? s : t) << endl;}
intl kai(intl k){
    intl a = 1;
    for(int i = 2; i <= k; i++){
        a *= i;
    }
    return a;
}
intl gcd(intl a, intl b){
    if(!b)return a;
    return gcd(b, a % b);
}
intl lcm(intl a,intl b){
    return a / gcd(a, b) * b;
}
intl digit10(intl a){
    intl b = 0;
    do{
        a /= 10;
        b++;
    }while(a);
    return b;
}
signed main(){
    cout << fixed << setprecision(10);
    intl n,m;
    cin >> n >> m;
    intl ans = m;
    rep(i,m - n + 1){
        ans -= i;
    }
    cout << -1 * ans << endl;
    return 0;
}