#include <bits/stdc++.h>
using namespace std;
int main() {
  int w, h, k, kolvo = 0;
  cin >> w >> h >> k;
  while (k > 0) {
    kolvo = kolvo + (h + (w - 1) + (h - 1) + (w - 2));
    k = k - 1;
    h = h - 4;
    w = w - 4;
  }
  cout << kolvo;
  return 0;
}
