#include <bits/stdc++.h>
using namespace std;
#define REP(i,n) for (int i = 0; i < n; i++)
typedef long long ll;
// Welcome to my source code!

int main() {
    int l, k;
    cin >> l >> k;
    if (l%k == 0) cout << (l/(k*2)-1)*k << endl;
    else cout << l/(k*2)*k << endl;
}