#include <bits/stdc++.h>
#define rep(i,n)for(long long i=0;i<(n);i++)
using namespace std;
typedef unsigned long long ull;
typedef long long ll;
const ll INF=1e18;
const int MOD=1e9+7;
const double pi=acos(-1);

int main(){
  ll l,k;
  cin >> l >> k;
  ll ans=k*(l/(2*k));
  if(l==2*k)ans=0;
  cout << ans << endl;
  
}