#include <iostream>

using namespace std;

int L, K;

void solve() {
    cin >> L >> K;
    int r = L % (K * 2);
    if (r == 0) cout << K * (L / (K * 2)) - K;
    else cout << K * (L / (K * 2));
    cout << endl;
}

int main() {
    solve();
    return 0;
}