import math


def polygon_area(n_sides, side_len):
    """Find the area of a regular polygon

    :param n_sides: number of sides
    :param side_len: length of polygon sides
    :return: area of polygon

    >>> round(polygon_area(4, 5))
    25
    """
    perimeter = n_sides * side_len

    apothem_denominator = 2 * math.tan(math.pi / n_sides)
    apothem = side_len / apothem_denominator

    return perimeter * apothem / 2