from fractions import Fraction

def urns(a):
    prob = [Fraction(x[0], sum(x)) for x in a]
    prob_c = [1 - x for x in prob]
    
    n = len(a)
    ans = 0
    for i in range(n):
        p = prob_c[i] * prob[(i + 1) % n] * prob[(i + 2) % n]
        ans += p
    return ans

b =  [[4, 3], [5, 4], [4, 4]]
print(urns(b))