There are tissue boxes. Their initial numbers of tissues are .
In one operation, let be the number of boxes that are currently empty. Choose a box containing at least tissues and remove exactly tissues from it.
You may perform any number of operations, including zero. Find the minimum total number of tissues that can remain in all boxes.
Before emptying a box, reduce the third box to one tissue and the fourth box to three tissues. The amounts are (1, 2, 1, 3). Then empty the first, second, and fourth boxes, removing one, two, and three tissues respectively. One tissue remains. Leaving zero is impossible: after two boxes become empty, any further emptying operation needs at least three tissues, but only the fourth box initially has that many.
While every operation still removes one tissue, reduce the first three boxes to one tissue each and the fifth box to three tissues. The amounts are (1, 1, 1, 2, 3). Empty the first, fourth, and fifth boxes by removing one, two, and three tissues respectively. Two tissues remain. At most two of the boxes that initially contain two tissues can become empty, because every emptying operation after the second needs at least three tissues. Hence, at least two tissues must remain.