import math


def _dif_angle(a, b, c):
    """
    Gets angle difference

    Parameters
    ----------
    a, b, c : tuples
        sequential node coords on projected graph.
    
    Returns
    -------
    float
    """
    ang = math.degrees(math.atan2(c[1]-b[1], c[0]-b[0]) - math.atan2(a[1]-b[1], a[0]-b[0]))
    return abs(ang) - 180 if abs(ang) > 180 else 180 - abs(ang)