def main():

  pocky_length, bite_length = map(int, input().split())
  bite_length_pair = bite_length * 2
  bite_count = pocky_length // bite_length_pair
  if bite_count > 1:
    ans = bite_count * bite_length
    print(ans)
  else:
    print(0)

if __name__ == '__main__':
  main()