from itertools import combinations
from fractions import Fraction

n = 3
k = 2

p_r = [Fraction(4,7), Fraction(5,9), Fraction(4,8)]
p_b = [Fraction(3,7), Fraction(4,9), Fraction(4,8)]
total = Fraction(0,1)

for i,j in combinations(range(n), k):
    remaining_option = next(x for x in range(n) if x not in (i, j))
    total += p_r[i] * p_r[j] * p_b[remaining_option]

print(total)