#include <bits/stdc++.h>

using namespace std;

#define int long long

signed main() {
  int n, d;
  cin >> n >> d;
  int m = n-d-1;
  if(d == 1) cout << n*(n-1)/2 << endl;
  else if(d == 2) cout << n*(n-1)/2-1 << endl;
  else cout << d+m*(m+1)/2+2*m << endl;
  return 0;
}

