#include <iostream>
using namespace std;

class Pockey
{
	int l;
	int k;
	int r;
public:
	Pockey(int ll,int kk);
	void Taberu();
	int Nagasa();
	int Tabeta();
};

Pockey::Pockey(int ll,int kk){
	l = ll;
	k = kk;
	r = 0;
}

void Pockey::Taberu(){
	l -= 2 * k;
	r += k;
}

int Pockey::Nagasa(){
	return l;
}

int Pockey::Tabeta(){
	return r;
}


int main()
{
	int l;
	int k;
	cin>>l;
	cin>>k;
	Pockey p(l,k);

	while(1){
		p.Taberu();
		if(p.Nagasa() <= 0){
			break;
		}
	}
	cout<<p.Tabeta()-k<<endl;


}