1#include<bits/stdc++.h>
2using namespace std;
3int main()
4{
5 unsigned int a, b;
6 long long int sum = 0, n;
7 double s1;
8 cin >> a >> b;
9 n = (b-a) + 1;
10 sum = n * ((double)(a+b) / 2);
11 cout << sum << endl;
12 return 0;
13}