#include<iostream>

using namespace std;

int sum(int l, int r)
{
    int sum = l;
    for(int i = l + 1;i <= r;i ++) sum += i;

    return sum;
}

int main()
{
    int l, r;
    cin >> l >> r;

    cout << sum(l, r) << endl;

    return 0;

}