#include <bits/stdc++.h>
#include <vector>
#include <iostream>
#include <map>
#include <string>
#include <cmath>
#include <algorithm>
#include <queue>
#include <math.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repe(i,x,n) for(int i=x; i<(n); ++i)
using ll = long long;
using P = pair<int, int>;
#define pai 3.1415926535897932384
#define INF 100100100
#define _GLIBCXX_DEBUG
#define All(a) (a).begin(),(a).end()
#define rAll(a) (a).rbegin(),(a).rend()
#define pb push_back
#define mk make_pair
#define yes cout << "Yes" << endl;
#define no cout << "No" << endl;
#define mod 998244353
#define mod2 1000000007
#define deci cout << fixed << setprecision(10);
#define nextP next_permutation
#define em emplace_back

int main() {

    ll L,K;
    cin >> L >> K;
    ll len = 0;
    if(L<=2*K) len = 0;
    else {
        ll na = 2*K;
        ll cn = L/na;
        len = K*cn;
    }
    cout << len << endl;
    return 0;

}