#include "bits/stdc++.h"
using namespace std;
#define Would
#define you
const int INF = 999999999;
const int MOD = 1e9 + 7;
const double pi = 3.141592653589793238;

int main() {
	long long a, b, ans;
	cin >> a >> b;
	if (b == 0 || b > a) { ans = 0; }
	else if (a % 2 == 0) { ans = a - 2; }
	else {
		if (2 * b == a + 1) { ans = a - 1; }
		else { ans = a - 2; }
	}
	cout << ans << endl;
}