Create a function to optimize a given python code using the Numba library.
def calculate_pi():
    c = 0.0
    for k in range(1000):
        for j in range(1000):
            c += k ** -2 + j ** -3
    return c
import numba

@numba.jit
def calculate_pi():
    c = 0.0
    for k in range(1000):
        for j in range(1000):
            c += k ** -2 + j ** -3
    return c