#include<iostream>
#include<vector>
#include<map>
#include<algorithm>
#include<math.h>
#include<string>
typedef long long ll;
const ll LINF = 1e18;
const int INF = 1e8;
using namespace std;

#define vvint(vec,n,m,l) vector<vector<int>> vec(n, vector<int>(m,l));    // lで初期化
#define vvll(vec,n,m,l) vector<vector<ll>> vec(n,vector<ll>(m,l));
#define vint vector<int>
#define pint pair<int,int>
#define rep(i,a) for(int i=0;i<(a);i++)
#define all(x) (x).begin(),(x).end()
#define debug system("pause")

int main(void)
{
    
    ll l;
    int k;

    cin>>l>>k;

    int ans=0;

    while (true)
    {
        if(l<=2*k)
        {
            break;
        }
        ans+=k;
        l-=2*k;
    }
    
    cout<<ans<<endl;

}