#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<pii,int> ppii;
typedef pair<ll,ll> pll;
typedef tuple<ll,ll,ll> tl;
ll mod=1000000007;
ll gcd(ll a,ll b){
  if(a<b)swap(a,b);
  if(a%b==0)return b;
  return gcd(b,a%b);
}
ll lcm(ll a,ll b){
  ll ret=a*b;
  ll u=gcd(a,b);
  return ret/u;
}
int main(){
  ll n,k;cin>>n>>k;
  ll u=((n+1)*n)/2;
  ll sum=0;
  for(int i=n;i>0;i--){
    if(i>n-k)sum-=i;
    else sum+=i;
  }
  cout<<(u-sum)/2+1<<endl;
}