#include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
#include<vector>
#include<algorithm>
#include<set>

using namespace std;

#define int long long
#define rep(i,n) for(int i = 0; i < (n); i++)
#define INF ((long long)1e18)
#define MOD ((int)1e9+7)
#define endl "\n"

#define yn(f) ((f)?"Yes":"No")
#define YN(f) ((f)?"YES":"NO")

#define MAX

signed main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout<<fixed<<setprecision(10);
	
	int N, K;
	int ans = 0;
	
	cin>>N>>K;
	
	ans = N*(N+1)/2 -((N-K)*((N-K)+1)/2 - K*(N+(N-K+1))/2);
	// cout<< N*(N+1)/2<<" "<<(N-K)*((N-K)+1)/2 <<" "<<K*(N+(N-K+1))/2<<endl;
	
	cout<<ans/2 + 1<<endl;
	
	return 0;
}