#include <bits/stdc++.h>
using namespace std;
using ll=long long;

ll downfloor(ll x,ll m){
  // x/mを正確に求める関数
  ll r=(x%m+m)%m;
  return (x-r)/m;
}

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