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  1. Prepare
  2. Python
  3. Itertools
  4. Iterables and Iterators
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Iterables and Iterators

  • bojan1997
     + 0 comments
    # Enter your code here. Read input from STDIN. Print output to STDOUT
from itertools import combinations


if __name__ == '__main__':
    n = int(input().rstrip())
    letters = input().rstrip().split()
    k = int(input().rstrip())
    combs = list(combinations(letters, k))
    num_of_a = 0
    for c in combs:
        if 'a' in c:
            num_of_a += 1
    print(round(num_of_a / len(combs), 12))