(a+b)!/a!b!

def perms(a, b):
    """ Computes (a+b)!/(a!b!) """
    a, b = min(a,b), max(a,b)
    prod = 1
    a_lst = [i for i in range(a, 0, -1)]
    for c in range(a+b, b, -1):
        a_lst_temp = list(a_lst)
        for i in a_lst_temp:
            if c % i == 0:
                c //= i
                a_lst.remove(i)
        prod *= c
    return prod