#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

#define INF 1000000000
#define EPS 1e-9
#define PI acos(-1)

typedef long long ll;
typedef pair<int, int> P;

ll L;
int K;

int main(){

  cin >> L >> K;
  
  ll num;
  num = L/(2*K);
  if(L%(2*K) == 0)
    num--;
  cout << num * K << endl;

  return 0;

}