#include<iostream>
#include<algorithm>
#include <vector>
#include<cmath>
#include<iomanip>
using namespace std;
typedef long long int lont;
int main() {
	int L;
	int K;
	cin >> L >> K;
	if (L % (2 * K) == 0) {
		cout << (L / (2 * K) - 1) * K << endl;
	}
	else {
		cout << L / (2 * K) * K;
	}
}