CF891E Lust

A false witness that speaketh lies! You are given a sequence containing $ n $ integers. There is a variable $ res $ that is equal to $ 0 $ initially. The following process repeats $ k $ times. Choose an index from $ 1 $ to $ n $ uniformly at random. Name it $ x $ . Add to $ res $ the multiply of all $ a_{i} $ 's such that $ 1

The first line contains two integers $ n $ and $ k $ ( $ 1

Output a single integer — the value .