На сколько изменится длина окружности, если радиус изменится на 1 мм?

My dear friend!
Sorry, I haven't been writing for ages. Had problems with my exams. But now I'm free from school and can spend more time chatting with you.
In last letter you asked me if I like sports. Yes, I like it all especially white water drafting and skydiving. I like it because it's dangerous and really cool. Do you like this sports?
Tell me about sports you like. Waiting for your answer,
Denis xx