ubunu 18.04 npmをインストールできない解決策

ubunu 18.04 npmプロンプトをインストールできません:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 npm : Depends: node-gyp (>= 0.10.9) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

次のコマンドを使用してインストールします.

sudo apt install aptitude
sudo aptitude install npm

このオプションに遭遇しました.

     Keep the following packages at their current version:
1)     libssl-dev [Not Installed]                         
2)     node-gyp [Not Installed]                           
3)     nodejs-dev [Not Installed]                         
4)     npm [Not Installed]     

Accept this solution? [Y/n/q/?] ?

yを選んだら、何の役にも立たないから、先に選んで?nを選択してインストールに成功しましたが、なぜか分かりません.

$ npm -v
3.5.2