#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#define REP(i,n) for(int i=0;i<(n);++i)
#define SORT(a) sort((a).begin(),(a).end())
#define PB push_back
using namespace std;
typedef vector<int> VI;

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