#include <bits/stdc++.h>

using namespace std;

int main(void) {
	long long a, b;
	cin >> a >> b;
	
	if (b >= 1 && b <= a) {
		if (a % 2 && (a + 1) / 2 == b) cout << max(0LL, a - 1) << endl;
		else cout << max(0LL, a - 2) << endl;
	}
	else cout << 0 << endl;
	return 0;
}