Goal: Learn to get substrings.

Assignment: In a system designed for a digital library archive, each document is tagged with a unique identifier. The identifier is split into three parts, each separated by a : character. The middle code represents the type of the document. The three parts are always three characters long each. The resulting document id is always $11$ characters long.

Given a String variable documentId, declare the string variable typeId, and initialize it to the middle part of the document id$.$

For example, if documentId were ABC:$123$:XYz$,$ then typeId would be $123.$