If you use the standard cargo run, you will be met with an error that the compiler is unable to find extern crate num. This is down to cargo not knowing where the dependencies are held. At the first time of grabbing an external reference, Rust will update the list of available crates (the registry) and then download the ones required. To do this, the Cargo.toml file needs to be edited and the following code inserted:

[dependencies] 
num = "*"

Once this has been saved and the cargo run executed, you will see output like this: