#include <bits/stdc++.h>
#include <iostream>
#include <string>
using namespace std;

int main() {
long long l,k,kk=0;
cin>>l>>k;

if(l<=k+k){
	cout<<0<<endl;
	return 0;
}

while(l>k*2+kk){
kk=kk+k*2;
}
cout<<kk/2<<endl;

}

#950832