#include <bits/stdc++.h>
using namespace std;
#define repa(i,n) for(it i=0;i<(it)n;i++)
#define repb(i,n) for(it i=1;i<=(it)n;i++)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define moda 998244353LL
#define modb 1000000007LL
#define dai 2500000000000000000LL
#define sho -dai
#define eps 1e-14
using it=long long;
using un=unsigned long long;
using db=long double;
using st=string;
using ch=char;
using bo=bool;
using P=pair<it,it>;
using T=tuple<it,it,it>;
using vi=vector<it>;
using vd=vector<db>;
using vs=vector<st>;
using vc=vector<ch>;
using vb=vector<bo>;
using vp=vector<P>;
using sp=set<P>;
using ss=set<st>;
using si=set<it>;
using vvi=vector<vi>;
using vvs=vector<vs>;
using vvb=vector<vb>;
using vvc=vector<vc>;
using vvp=vector<vp>;
using vsi=vector<si>;
using vsp=vector<sp>;
using vvsi=vector<vsi>;
using vvvi=vector<vvi>;
using vvvvi=vector<vvvi>;
const it dx[4]={0,1,0,-1};
const it dy[4]={1,0,-1,0};

it floor(it x, it m) {
  it r=(x%m+m)%m;
  return (x-r)/m;
}

int main() {
  it l,k;cin>>l>>k;
  cout<<floor(l-1,k*2)*k<<endl;
}