#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <utility>

using namespace std;

int main(void){

    int l, k;
    cin >> l >> k;
    int ans = l / k;
    ans /= 2;
    if(l % k == 0 && (l / k) % 2 == 0) ans--;
    cout << ans * k << endl;

    return 0;
}