#include <iostream>
using namespace std;

int main() {
    int L, K;
    cin >> L >> K;
    if (L % (K * 2) == 0) {
        cout << ((L / (K * 2)) - 1) * K << endl;
        return 0;
    }
    cout << (L / (K * 2)) * K << endl;
    return 0;
}