import random
element, repeat = map(int, input().split())
numbers = []
for ele in element:
    numbers.append(ele)
    

sum_numbers = []
for _ in repeat-1:
    random_number = random.choice(numbers)
    numbers = map(lambda x: random_number + x, numbers)
        
for num in numbers:
    add_numbers = map(lambda x: random_number + x, numbers)
    sum_numbers.append(add_numbers)

print(sum(sum_numbers) / element)