#include<iostream>
#include<string>
#include<algorithm>
#include<cmath>
#include<iomanip>
#include<cstring>
#include<map>
#include<vector>
#include<queue>
#include<climits>
using namespace std;
typedef long long int ll;

int main(){

	int n, k;
	cin >> n >> k;
	if(n==1){
		cout << 0 << endl;
	} else if(n%2==0){
		if(k==0 || k>n){
			cout << 0 << endl;
		} else {
			cout << n-2 << endl;
		}
	} else {
		if(k==n/2+1){
			cout << n-1 << endl;
		} else if(k==0 || k>n){
			cout << 0 << endl;
		} else {
			cout << n-2 << endl;
		}
	}

  return 0;
}