def main():

  pocky_length, bite_length = map(int, input().split())
  bite_length_pair = bite_length * 2
  bite_count = pocky_length // bite_length_pair

  if pocky_length % bite_length_pair  == 0:
    bite_count = bite_count - 1
    ans = bite_count * bite_length
    print(ans)

  else:
    if bite_count >= 1:
      ans = bite_count * bite_length
      print(ans)
    else:
      print(0)

if __name__ == '__main__':
  main()