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 line y = ax + c, y = ax – d, y = ax – d, y = bx + c and y = bx – d has area 72. If a, b, c, d are positive integers then the least possible value of the sum a + b + c + d is