#include <bits/stdc++.h>
int main() {
  int w, h, k, r = 0;
  std::cin >> w >> h >> k;
  for (int i = 0; i < k; i++) {
    r = r + (w * 2 + (h - 2) * 2);
    w = w - 4;
    h = h - 4;
  }
  std::cout << r;
  return 0;
}
