#include <algorithm>
#include <cstdio>
#include <iostream>
#include <numeric>
#include <optional>
#include <vector>

using namespace std;

int main() {
    int32_t l, k;
    cin >> l >> k;
    auto ans = (l / 2 / k) * k;
    if (ans * 2 == l)
        ans -= k;
    cout << ans << endl;
    return 0;
}