KMO 2013 Problem 8

For positive integer $a,b,c,d$ there are $a+b+c+d$ points on plane which none of three are collinear. Prove there exist two lines $l_1, l_2$ such that (1) $l_1, l_2$ are not parallel. (2) $l_1, l_2$ do not pass through any of $a+b+c+d$ points. (3) There are $a, b, c, d$ points on each region separated by two lines $l_1, l_2$ .