#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

long long f(long long n) {
  return n*(n+1)/2;
}

int main() {
  long long N, K;
  cin >> N >> K;
  // 1 .. N-K, N-K+1, N
  long long R = f(N);
  long long L = -(f(N) - f(N-K)) + f(N-K);
  cout << (R-L)/2+1 << endl;
}