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

using namespace std;
typedef long long ll;
typedef pair<ll, ll> pl;

int main()
{
	int N, K;
	cin >> N >> K;

	ll ans = 0;
	if (N == 2 && K == 1)ans = 0;
	else {
		int num1 = N - K;
		int num2 = K - 1;
		ans = (num1*(num1 + 1)) / 2;
		if (num2 != 0)ans++;
	}
	cout << ans << endl;
	return	0;
}