#include <iostream>
using namespace std;
 
int main() {
	int H, N, tmp, answer = 0;
	cin >> H >> N;
	tmp = abs(H);
	for (int i = 0; i < N; i++) {
		answer += (i%2 == 0 ? 1 : -1) * tmp;
		tmp ++;
	}
	cout << answer << endl;
	return 0;
}