#include <iostream>
#include <vector>

using namespace std;
int main(){
	int l, k,a;
	cin >> l,cin >> k;
	if (l / (k * 2) == 1){
		a = 0;
	}
	else{
		a = (l / (k * 2)) * k;
	}
	cout << a << endl;
	return 0;
}