#include<string>
#include<iostream>
#include<algorithm>
using namespace std;
int main()
{
	int l,k;
	cin>>l>>k;
	int cnt = 0;
	while(1)
	{
		if(l-2*k > 0)
		{
			l -= 2*k;
			cnt+=k;
		}else break;
	}
	cout<<cnt<<endl;
	return 0;
}