#include <bits/stdc++.h>

using namespace std;

using ll = long long;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll n, k;
    cin >> n >> k;
    cout << (2 * n - k + 1) * k / 2 + 1 << endl; 
    return 0;
}