Calculate the given percentile of arr.

def percentile(arr, percent):
    """
    Calculate the given percentile of arr.
    """
    arr = sorted(arr)
    index = (len(arr) - 1) * percent
    floor = math.floor(index)
    ceil = math.ceil(index)
    if floor == ceil:
        return arr[int(index)]
    low_value = arr[int(floor)] * (ceil - index)
    high_value = arr[int(ceil)] * (index - floor)
    return low_value + high_value