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