#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>

using namespace std;


int main(){
	int l,k;
	cin >> l >> k;
	int ans = l/(2*k);
	if(l%(2*k) == 0) ans--;
	cout << k*ans << endl;
	return 0;
}