#include<iostream>
#include<algorithm>
#include <vector>
#include<cmath>
#include<iomanip>
#include<queue>
using namespace std;
typedef long long int lint;
int main() {
	int N, K;
	cin >> N >> K;
	if (K == 0 || K > N) {
		cout << 0 << endl;
	}
	else if (N % 2 == 1 && K == N / 2 + 1) {
		cout << N - 1 << endl;
	}
	else {
		cout << N - 2 << endl;
	}
}