#include <iostream>

using namespace std;

int solve(int n, int k) {
	
	if (k < 1 || k > n) {
		return 0;
	} else if (n % 2 == 0) {
		return n - 2;
	} else {
		if (k == n / 2 + 1) {
			return n - 1;
		} else {
			return n - 2;
		}
	}
}

int main () {
	
	int n, k;
	cin >> n >> k;
	
	int ret = solve(n, k);
	cout << ret << endl;
	
	return 0;
}