#include <iostream>

int main() {
	int n, k;
	std::cin >> n >> k;

	if (k == 0 || k > n) {
		std::cout << 0 << std::endl;
	} else if (k * 2 - 1 == n) {
		std::cout << n - 1 << std::endl;
	} else {
		std::cout << n - 2 << std::endl;
	}
}