#include <vector>
#include <queue>
#include <deque>
#include <stack>
#include <map>
#include <set>
#include <algorithm>
#include <functional>
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <cctype>
#include <string>
#include <cstring>

using namespace std;

typedef long long ll;


int main() {
  ll l, k;
  cin >> l >> k;
  if (l%(2*k)) {
    cout << (l/(2*k))*k << endl;
  }else {
    cout << max((l/(2*k)-1)*k, ll(0)) << endl;
  }
}