#line 2 "header.hpp"
 
//%snippet.set('header')%
//%snippet.fold()%
#ifndef HEADER_H
#define HEADER_H
 
// template version 2.0
using namespace std;
#include <bits/stdc++.h>
 
// varibable settings
const long long INF = 1e18;
template <class T> constexpr T inf = numeric_limits<T>::max() / 2.1;
 
#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (ll i = (ll)(a); i < (ll)(b); ++i)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)
#define _rrep(i, n) rrepi(i, 0, n)
#define rrepi(i, a, b) for (ll i = (ll)((b)-1); i >= (ll)(a); --i)
#define r_rep(...) _overload3(__VA_ARGS__, rrepi, _rrep, )(__VA_ARGS__)
#define each(i, a) for (auto &&i : a)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(...) make_tuple(__VA_ARGS__)
#define ub upper_bound
#define lb lower_bound
#define lpos(A, x) (lower_bound(all(A), x) - A.begin())
#define upos(A, x) (upper_bound(all(A), x) - A.begin())
template <class T> inline void chmax(T &a, const T &b) { if ((a) < (b)) (a) = (b); }
template <class T> inline void chmin(T &a, const T &b) { if ((a) > (b)) (a) = (b); }
template <typename X, typename T> auto make_table(X x, T a) { return vector<T>(x, a); }
template <typename X, typename Y, typename Z, typename... Zs> auto make_table(X x, Y y, Z z, Zs... zs) { auto cont = make_table(y, z, zs...); return vector<decltype(cont)>(x, cont); }
 
#define divceil(a, b) ((a) + (b)-1) / (b)
#define is_in(x, a, b) ((a) <= (x) && (x) < (b))
#define uni(x) sort(all(x)); x.erase(unique(all(x)), x.end())
#define slice(l, r) substr(l, r - l)
 
typedef long long ll;
typedef long double ld;
using vl = vector<ll>;
using vvl = vector<vl>;
using pll = pair<ll, ll>;
 
template <typename T>
using PQ = priority_queue<T, vector<T>, greater<T>>;
void check_input() { assert(cin.eof() == 0); int tmp; cin >> tmp; assert(cin.eof() == 1); }
 
#if defined(PCM) || defined(LOCAL)
#else
#define dump(...) ;
#define dump_1d(...) ;
#define dump_2d(...) ;
#define cerrendl ;
#endif
 
#endif /* HEADER_H */
//%snippet.end()%
#line 2 "solve.cpp"
template<class T=ll> using vec = vector<T>;
struct Fast { Fast() { std::cin.tie(0); ios::sync_with_stdio(false); } } fast;
 
// snippet:segment_tree {{{
template <typename T> struct SegmentTree {  // {{{
    private:
        using F = function<T(T, T)>;
        int n;  // 元の配列のサイズ
        int N;  // n以上の最小の2冪
        vector<T> node;
        F merge;
        T identity;
 
    public:
        SegmentTree() {}
        SegmentTree(vector<T> a, F f, T id) : merge(f), identity(id) {
            n = a.size();
            N = 1;
            while (N < n) N *= 2;
            node.resize(2 * N - 1, identity);
            for (int i = 0; i < n; i++) node[i + N - 1] = a[i];
            for (int i = N - 2; i >= 0; i--)
                node[i] = merge(node[2 * i + 1], node[2 * i + 2]);
        }
        SegmentTree(int n, F f, T id) : SegmentTree(vector<T>(n, id), f, id) {}
 
        T& operator[](int i) { return node[i + N - 1]; }
 
        void update(int i, T val) {
            i += (N - 1);
            node[i] = val;
            while (i > 0) {
                i = (i - 1) / 2;
                node[i] = merge(node[2 * i + 1], node[2 * i + 2]);
            }
        }
 
        void add(int i, T val) {
            i += (N - 1);
            node[i] += val;
            while (i > 0) {
                i = (i - 1) / 2;
                node[i] = merge(node[2 * i + 1], node[2 * i + 2]);
            }
        }
 
        // query for [l, r)
        T query(int a, int b, int k = 0, int l = 0, int r = -1) {
            if (r < 0) r = N;
            if (r <= a || b <= l) return identity;
            if (a <= l && r <= b) return node[k];
 
            T vl = query(a, b, 2 * k + 1, l, (l + r) / 2);
            T vr = query(a, b, 2 * k + 2, (l + r) / 2, r);
            return merge(vl, vr);
        }
 
#if defined(PCM) || defined(LOCAL)
        friend ostream& operator<<(ostream& os, SegmentTree<T>& sg) {  //
            os << "[";
            for (int i = 0; i < sg.n; i++) {
                os << sg[i] << (i == sg.n - 1 ? "]\n" : ", ");
            }
            return os;
        }
#endif
};/*}}}*/
/*
int solve() {
    ll n;cin>>n;
    vector<ll> a(n+1);
    rep(i, 1, n+1) { cin>>a[i]; }
    dump(a);
 
    vl b(a);
    vec<pll> c(n+1);
    rep(i, 1, n+1){
        b[i] += b[i-1];
        c[i] = mp(b[i], -i);
    }
    dump(b);
 
    auto mymax=[](auto a, auto b){return max(a,b);};
 
    a[0] = -INF;
    SegmentTree<ll> seg2(a, mymax, -1e18);
    SegmentTree<pll> seg(c, mymax, mp(-1e18, -1e18));
 
    ll ans = 0;
    rep(l, 1, n+1){
        auto [s, idx] = seg.query(l, n+1);
        idx *= -1;
        ll max_el = seg2.query(l, idx+1);
        dump(s, idx, max_el);
        chmax(ans, b[idx] - b[l-1] - max_el);
    }
    cout << ans << endl;
 
    return 0; 
}
*/
 
int main(){/*{{{*/
    int a,b;
    cin>>a>>b;
    int n=b-a;
    int ans=-a;
    rep(i,n){
        ans+=i;
    }
    cout<<ans;
    return 0;
}/*}}}*/

Measured memory_bytes: 256