algebra precalculus - Find the value of $cos 1^circ + cos 2^circ + cos 3^circ +.....+cos 180^circ$

Find the value of $\cos 1^\circ + \cos 2^\circ + \cos 3^\circ +.....+\cos 180^\circ$

My Attempt:

$$=\cos 1^\circ + \cos 2^\circ + \cos 3^\circ +....+\cos 180^\circ$$
$$= \cos 1^\circ + \cos 2^\circ + \cos 3^\circ +....-1$$

How do I complete it?