Specify input formats for each task in lesson 6

[AbdullinAM]

Lesson 6 is partly dedicated for checking students ability to use regular expressions to check the format of input data. However, we never specify what kind of symbols they may encounter in the input and how they should handle it. One of the examples if mostExpensive from lesson 6. We never specify the format of product name: is it only Russian, or may be English, can it have whitespaces etc. This causes a lot of confusion for students