#include <iostream>

using namespace std;

int main(){
	long int n, k;
	cin >> n >> k;
	if (k == 0 || k > n || (n==1 && k==1) || (n==2 && k==2)) cout << 0 << endl;
	else{
		if (k != n-k+1) cout << n - 2 << endl;
		else cout << n-1 << endl;
	}

}