#define _USE_MATH_DEFINES
#include<iostream>
#include<vector>
#include<algorithm>
#include<cmath>
#include<string>
#include<iomanip>
#include<numeric>
#include<queue>
#include<deque>
#include<stack>
#include<set>
#include<map>
#include<random>
using namespace std;
typedef long long ll;
const int mod=1e9+7;
int main(){
	int n,k;
	cin>>n>>k;
	cout<<(k==0||k>n?0:n%2==1&&k==(n+1)/2?n-1:n-2)<<endl;
}