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Iterables and Iterators

  • shagun_jain_
     + 0 comments
    from math import factorial as fac

n=int(input())
list=input().split()
m=int(input())

# total events: nCm combination formula
total=fac(n)/(fac(m)*fac(n-m))

# events when selected indices doesn't contains "a"
n_dup=len(list)-list.count('a')

# if selecting indices is not possible without picking'a'
if n_dup<m:
    result=1
else:
    prob_bar=fac(n_dup)/(fac(m)*fac(n_dup-m))
    result=(total-prob_bar)/total
    
print(result)