include[HTML_REMOVED]

using namespace std;
int sum(int l, int r)
{
int ans=0;
cin >> l >> r;
for(int i=l;i<=r;i++)
{
ans+=i;
}
return ans;
}

int main()
{
int l,r;
cin >> l >> r;
cout << sum(l,r) << endl;
return 0;
}