#include <bits/stdc++.h>
using namespace std;

int main() {
    ios::sync_with_stdio(0);
    cin.tie(nullptr);
    long long a, b;
    cin >> a >> b;
    
    auto f = [&] (long long x) { return (x * b * 2 < a); };
    
    long long ok = 0, ng = 1e12;
    while (abs(ok - ng) > 1) {
        long long x = (ok + ng) / 2;
        if (f(x)) ok = x;
        else ng = x;
    }
    
    cout << b * ok << '\n';
    return 0;
}