#include <iostream>
#include <list>
#include <vector>
#include <string>
#include <stdio.h>
#include <stdint.h>
using namespace std;

using uint = unsigned int;
using ll = long long;

#define CIN( LL , A ) LL A; cin >> A 

int main()
{

  CIN( ll , L );
  CIN( ll , K );
  const ll K2 = K * 2;
  
  cout << ( L / K2 - ( L % K2 == 0 ? 1 : 0 ) ) * K << endl;
  return 0;

}