#include<iostream>

using namespace std;

int main() {

	int n, k;
	cin >> n >> k;

	if (k == 0 || k > n)cout << 0 << endl;
	else if (n % 2 == 0)cout << n - 2 << endl;
	else {
		if (k == (n + 1) / 2)cout << n - 1 << endl;
		else cout << n - 2 << endl;
	}

	return 0;
}