#include <iostream>
using namespace std;

int main()
{
	int L, K;

	cin >> L >> K;

	int sub = K;

	while(L > sub * 2)
	{	
		/*if (L / 2 <= sub)
		{
			cout << sub - K << endl;
			break;
		}*/

		sub += K;

	}

	cout << sub - K << endl;

	return 0;
}