#include <algorithm>
#include <numeric>
#include <string>
#include <vector>
#include <iostream>
#include <queue>
#include <utility>
#include <cmath>
using namespace std;

using ull = unsigned long long;

int main()
{
    ull l,k;
    cin >> l >> k;

    ull num = 0;
    if(l%(2*k) == 0)
    {
        num = l/(2*k);
        cout << (num -1)*k << endl;
    }
    else
    {
        num = l/(2*k);
        cout << num*k << endl;
    }
}