#include <bits/stdc++.h>
using namespace std;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0); 
	
	int n, k;
	cin >> n >> k;
	if(k > n || k == 0) cout << 0 << endl;
	else {
		if(n % 2 == 1) {
			if(k == (n + 1) / 2) cout << n - 1 << endl;
			else cout << n - 2 << endl;
		} else {
			cout << n - 2 << endl;
		}
	}
	return 0;
}