#include<iostream>

using namespace std;

int main(){
	int nagasa,ryou,tabeta;
	int i;

	cin >> nagasa >> ryou;

	for(i = 0;;i++){
		if(nagasa <= ryou*2){
			break;
		}
		if(nagasa > ryou*2){
			nagasa -= ryou*2;
		}

		tabeta += ryou;
	}

	cout << tabeta << endl;

	return 0;
}