#include <bits/stdc++.h>
using namespace std;

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