#include <bits/stdc++.h>
using namespace std;
using lint = long long;
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V< V<T> >;

int main() {
  cin.tie(nullptr); ios::sync_with_stdio(false);
  lint n, k; cin >> n >> k;
  k = min(k, n);
  lint r = n * (n + 1) / 2;
  lint l = (n - k) * (n - k + 1) / 2 - (2 * n - k + 1) * k / 2;
  cout << (r - l) / 2 + 1 << '\n';
}