Six dice are stacked on the floor as shown in the figure below. On each dice, the sum of numbers on opposite faces is 7, i.e., if 1 written on one face then 6 is written on the face opposite it and so on. [Figure: six dice stacked in steps on the floor - a column of three dice on the left, two dice in the middle column and one die on the right, 21 faces visible] What is the maximum possible sum of numbers on the 21 visible faces?