#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr), cout.tie(nullptr);
  int n, v;
  cin >> n >> v;
  if (n - 1 <= v) {
    cout << n - 1 << endl;
    return 0;
  }
  int result = v - 1;
  for (int i = 1; i <= n - v; ++i) {
    result += i;
  }
  cout << result << endl;
  return 0;
}
