#include <bits/stdc++.h>
void answer(size_t v) { std::cout << v << '\n'; }
void solve(size_t w, size_t h, size_t k) {
  size_t count = 0;
  for (size_t i = 0; i < k; ++i) {
    count += 2 * (h + w - 2);
    h -= 4, w -= 4;
  }
  answer(count);
}
int main() {
  size_t w, h, k;
  std::cin >> w >> h >> k;
  solve(w, h, k);
  return 0;
}
