optimizing

Write a function optimizing:

• input parameter: an int-type object n satisfying $0\le$n$<2^{63}$
• return value: the int-type object f(n) where f is the following Python function:

def f(n):
    assert type(n)==int and n>=0
    g=lambda n:sum([x**3 for x in range(n)])
    x=""
    k=0
    while n>=len(x):
        x+=str(g(k))
        k+=1
    return int(x[n])