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