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


int main()
{
    int l, k;
    cin >> l >> k;
    
    int tmp = l / (2 * k);
    
    int ans = k * tmp;
    
    //cout << tmp << "\n";
    //cout << l - ans << "\n";
    
    if (l - 2*ans > 0 && tmp != 0) cout << ans << "\n";
    else cout << 0 << "\n";
    
    return 0;
    
}