#include <bits/stdc++.h>
using namespace std;
 
typedef long long ll;
typedef pair<int,int> P;
 
const int INF = 1e9;
const int mod = 1e9+7;
const double EPS = 1e-10;
const double PI = acos(-1.0);
 
int main()
{
    int l,k;
    cin >> l >> k;
    int ans = k*(l/(2*k));
    if(l%(2*k) == 0) ans -= k;
    cout << ans << endl;
    return 0;
}