def percentile(sorted_list, percent, key=lambda x: x):
    if not sorted_list:
        return None
    if percent == 1:
        return float(sorted_list[-1])
    if percent == 0:
        return float(sorted_list[0])
    n = len(sorted_list)
    i = percent * n
    if ceil(i) == i:
        i = int(i)
        return (sorted_list[i-1] + sorted_list[i]) / 2
    return float(sorted_list[ceil(i)-1])