#include <iostream> 
#include <string> 
#include <vector> 
#include <cmath> 
#include <algorithm> 
#include <cstdlib> 
#include <ctime> 
#include <cstdio> 
#include <functional> 
#include <set> 
#include <sstream> 
#include <cctype>

using namespace std; 


int main(){

	long n,k;
	cin>>n>>k;

	long num=n/(2*k);
	if(n%(2*k)==0) num--;

	cout<<num*k<<endl;


	return 0;
}