# include <iostream>
# include <algorithm>
# include <vector>
# include <string>
# include <set>
# include <map>
# include <cmath>
# include <iomanip>
# include <functional>
# include <utility>
# include <stack>
# include <queue>
# include <list>
# include <bitset>
using namespace std;
using LL = long long;
using ULL = unsigned long long;
constexpr long long MOD = 1000000000 + 7;
constexpr long long INF = 1000000000;
const double PI = acos(-1);

int main() {
	int l, k;
	cin >> l >> k;
	cout << (l % (k * 2) != 0 ? k*(l / (k * 2)) : k*(l / (k * 2) - 1)) << endl;
}