import math


def percentile(N, percent, key=lambda x: x):
    """
    Find the percentile of a list of values.
    @parameter N - is a list of values. Note N MUST BE already sorted.
    @parameter percent - a float value from 0.0 to 1.0.
    @parameter key - optional key function to compute value from each element of N.
    @return - the percentile of the values
    """
    if not N:
        return None
    k = (len(N) - 1) * percent
    f = math.floor(k)
    c = math.ceil(k)
    if f == c:
        return key(N[int(k)])
    d0 = key(N[int(f)]) * (c - k)
    d1 = key(N[int(c)]) * (k - f)
    return d0 + d1