def interpolate(x, y, x1):
    """
	Returns y1 that is the interpolated y value for x1, given two lists of
	x and y values.

    Example:

    if the x values are:
	x = [13000, 13500]
    and the corresponding y values are:

	y = [2000, 25O0]
	and we want to know the interpolated y value for x = 13100: 
    >>> x = [13000, 13500]
    >>> y = [2000, 2500]
    >>> x1 = 13100
    >>> interpolate(x, y, x1)
    2100.0
	"""
    for item in x:
        item = float(item)
    for item in y:
        item = float(item)
    x1 = float(x1)
    y1 = y[0] + (x1 - x[0]) / (x[1] - x[0]) * (y[1] - y[0])
    return y1
