def main():

  bite_count = 0
  pocky_length, bite_length = map(int, input().split())
  bite_length_pair = bite_length * 2
  while pocky_length > bite_length_pair:
    pocky_length -= bite_length_pair
    bite_count += 1
  print(bite_count * bite_length)

if __name__ == '__main__':
  main()