#include <iostream>
#include <vector>
#include <map>
#include<string>
#include<queue>
#include<math.h>
#define all(x) begin(x),end(x)
#define ll long long
#define P pair<int,int>
#define lP pait<ll,ll>
using namespace std;
int main() {
    int l,k;
    cin>>l>>k;
    if(l%(k*2)==0) cout<<(l/(k*2)-1)*k<<endl;
    else cout<<(l/(k*2))*k<<endl;
    return 0;
}