from typing import Literal
import pandas as pd
import numpy as np

def get_first_valid_return_index(series: pd.Series) -> int:
    return np.where(np.logical_and(series != 0, np.logical_not(np.isnan(series))))[0][0]

def flatten(list_of_lists: list) -> list:
    return [item for sublist in list_of_lists for item in sublist]