#include <vector>
#include <iostream>
#include <string>
#include <sstream>
#include <algorithm>
#include <utility>
#include <map>

using namespace std;
typedef long long ll;
typedef pair<ll,ll> P;

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