The quadrilateral formed by the lines y=ax+c, y=ax+d, y=bx+c and y=bx+d has area 18. The quadrilateral formed by the lines y=ax+c, y=ax-d, y=bx+c and y=bx-d has area 72. Ifa,b,c,d are positive integers then the least possible value of the sum a+b+c+d is