#include <iostream>	//cin, cout
#include <vector>	//vector
#include <algorithm> //sort,min,max
#include <string>	//string
#include <ios>		//fixed
#include <iomanip>	//setprecision
#include <utility> //swap
#include <cstdlib>	//abs(int)

using namespace std;

int main() {

	int L, K;
	cin >> L >> K;

	int a = L / (K * 2);
	int b = L % (K * 2);

	int ans = 0;

	if (b != 0) {
		ans = a * K;
	}
	else {
		ans = (a - 1) * K;
	}

	cout << ans << endl;

	return 0;

}