Area of square: $$S_s = L*L$$
Area of circle: $$S_c = Pi*(L/2)2$$
Draw random dots, with sufficient dots, the probability of dots appearing in the circle approaximately equals the area of the circle within the area of the square:
$$N_c ⁄ N_c ≈ S_c ⁄ S_s = (Pi*(L/2)2) ⁄ (L*L)$$
$$=> Pi = N_c ⁄ N_s *4 $$
Play the demo here
Code in Github