acpc2016day2E.cpp

signed main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

    int n, d;
    cin >> n >> d;

    int ans;
    if (d == 1) {
        ans = n * (n - 1) / 2;
    } else {
        ans = (n - 1) + (n - d - 1) * n - (n - d - 1) * (n + d - 2) / 2;
    }
    cout << ans << endl;
}