CF1031A Golden Plate

You have a plate and you want to add some gilding to it. The plate is a rectangle that we split into $ w\times h $ cells. There should be $ k $ gilded rings, the first one should go along the edge of the plate, the second one — $ 2 $ cells away from the edge and so on. Each ring has a width of $ 1 $ cell. Formally, the $ i $ -th of these rings should consist of all bordering cells on the inner rectangle of size $ (w - 4(i - 1))\times(h - 4(i - 1)) $ . The picture corresponds to the third example.Your task is to compute the number of cells to be gilded.

The only line contains three integers $ w $ , $ h $ and $ k $ ( $ 3 \le w, h \le 100 $ , $ 1 \le k \le \left\lfloor \frac{min(n, m) + 1}{4}\right\rfloor $ , where $ \lfloor x \rfloor $ denotes the number $ x $ rounded down) — the number of rows, columns and the number of rings, respectively.

Print a single positive integer — the number of cells to be gilded.

The first example is shown on the picture below. The second example is shown on the picture below. The third example is shown in the problem description.