I am trying to convert exponential form to a number but I can't figure out how it has to be done.

I have a number `1820674000000385406`.

I am pasting this number excel after saving this I am following these steps
Format cells->Number and selecting the type.

but after doing this I am not getting my original number I am getting the number as `1820674000000380000`

but I want my original number back. Please help me with this.