#include<bits/stdc++.h>

using namespace std;

#define pb push_back
#define pf push_front
#define mp make_pair
#define fi first
#define se second
#define All(s) s.begin(),s.end()
#define rAll(s) s.rbegin(),s.rend()
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, a, n) for(int i = (a); i < (n); i++)
#define repr(i, n) for(int i = (n); i >= 0; i--)
#define repr2(i, a, n) for(int i = (n); i >= (a); i--)

template<typename T,typename U>
void Inparray(T &f,U n){
  for(int i=0;i<n;i++) cin>>f[i];
}

template<typename T,typename U>
void Printarray(T f,U n){
  for(int i=0;i<n;i++) i!=n ? cout<<f[i]<<" " : cout<<f[i];
}

void flagfunc(bool flag,string A,string B){
  flag ? cout<<A<<endl : cout<<B<<endl;
}

void flag2func(bool flag){
  flag ? cout<<"Yes"<<endl : cout<<"No"<<endl;
}

void flag3func(bool flag){
  flag ? cout<<"YES"<<endl : cout<<"NO"<<endl;
}

int main(){
  long long l,k;cin>>l>>k;
  int ans=0;
  while(1){
    if(l<=k*2)break;
    l-=k*2;
    ans+=k;
  }
  cout<<ans<<endl;
}