#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
 
#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
 
#include <functional>
#include <cassert>
 
typedef long long ll;
using namespace std;
 
#define debug(x) cerr << #x << " = " << x << endl;
 
 
#define mod 1000000007 //1e9+7(prime number)
#define INF 1000000000 //1e9
#define LLINF 2000000000000000000LL //2e18
#define SIZE 10000
 
 
int main(){
  ll n,d;
  cin >> n >> d;
 
  ll ans = 0;
   
  if(d == 1){
    ans = n*(n-1)/2;
  }else{
    ans = d + (n-d-1)*(n-d-2)/2+(n-d-1)*3;
  }
 
  cout << ans << endl;
   
  return 0;
}