#include<iostream>
#include<vector>
#include<cmath>

int main() {
	int n, k, count = 1;

	std::cin >> n >> k;

	if (k == 1) {
		std::cout << 0 << std::endl;
		return 0;
	}

	for (int i = k + 1; i <= n; i++) {
		count += n - i + 1;
	}

	std::cout << count << std::endl;
	
	return 0;
}