#include <iostream>

using lint = long long;

void solve() {
    lint n, k;
    std::cin >> n >> k;
    lint m = n - k;
    std::cout << n * (n + 1) / 2 - m * (m + 1) / 2 + 1 << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}