#include <iostream>
#include <string>
#include <algorithm>
#include <cstdio>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <numeric>
#include <cmath>

using namespace std;

typedef long long int ll;
typedef pair<int,int> P;

#define all(x) x.begin(),x.end()

const ll mod = 1e9+7;
const ll INF = 1e9;
const ll MAXN = 1e9;

int main()
{
	ll n,k;
	cin>>n>>k;
	ll sum = n*(n+1)/2;
	ll sum_k  = sum - (n-k)*(n-k+1)/2;
	ll mi = sum - 2LL*sum_k;
	cout<<(sum-mi)/2LL+1<<endl;

	return 0;
}