Using a C ++ static library (.lib) in a C # project?

I am trying to do a C # project and I need to use a solver I found on the internet to do some calculations, but this solver is a static library that writes in C ++.

it is so difficult to rewrite this library in .Net and it will take too long.

I've read some articles about what I like to do managed C ++, but I don't know what is the simplest solution to use this .lib in a C # project?

this is a static library