#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, v;
  cin >> n >> v;
  vector<vector<int> > mat(n + 1, vector<int>(v + 1, INT_MAX));
  for (int i = 1, j = 0; j <= v; j++) {
    mat[i][j] = j;
  }
  for (int i = 2; i <= n; i++) {
    for (int j = 0; j <= v; j++) {
      if (j < v) {
        mat[i][j] = mat[i - 1][j + 1];
      } else {
        mat[i][j] = mat[i][j - 1] + i;
      }
    }
  }
  cout << mat[n][0] << endl;
  return 0;
}
