#include <bits/stdc++.h>
using namespace std;
typedef unsigned long long ul;
typedef signed long long ll;
ul over = 1000000007;

int main(void)
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed;
  ll l, k;
  cin >> l >> k;
  ll c = (l-1)/(2*k);
  cout << c*k << endl;
  return 0;
}