#include <algorithm>
#include <iostream>
#include <iomanip>
#include <climits>
#include <string>
#include <vector>
#include <cmath>
#include <set>
#include <map>
using namespace std;
using ll = long long;

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