#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <cmath>
#include <iomanip>
#include <map>
using namespace std;
typedef long long LL;

int main(){
    LL L, K;
    cin >> L >> K;
    cout << (L-1) / (2*K) * K << endl;
    return 0;
}