/**
*    author:  boutarou
*    created: 02.08.2020 11:49:27
**/

#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < int(n); i++)
using ll = long long;
using P = pair<int, int>;

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