import math


def calculate_angle(v):
    """ Finds the angle of the two 2-d vectors in radians

    Parameters
    ----------
    v : tuple
        vector

    Returns
    -------
    angle of vector in radians
    """
    if v[0] == 0:
        return math.pi / 2 if v[1] > 0 else 3 * math.pi / 2

    angle = math.atan(v[1] / v[0])

    if v[0] > 0:
        return angle if angle >= 0 else 2 * math.pi + angle
    else:
        return angle + math.pi if angle >= 0 else (2 * math.pi + angle) - math.pi