#include <bits/stdc++.h>
using namespace std;
int ROW[] = {+1, -1, +0, +0};
int COL[] = {+0, +0, +1, -1};
int X[] = {+0, +0, +1, -1, -1, +1, -1, +1};
int Y[] = {-1, +1, +0, +0, +1, +1, -1, -1};
int KX[] = {-2, -2, -1, -1, 1, 1, 2, 2};
int KY[] = {-1, 1, -2, 2, -2, 2, -1, 1};
int GCD(int a, int b) { return b == 0 ? a : GCD(b, a % b); }
int LCM(int a, int b) { return a * (b / GCD(a, b)); }
bool CMP(int a, int b) { return a > b; }
int main() {
  int n, v;
  cin >> n >> v;
  n--;
  int ans = 0, f;
  ans += min(n, v);
  f = ans;
  int i = 2;
  while (f < n) {
    ans += i;
    f++;
    i++;
  }
  cout << ans << endl;
  return 0;
}
