#include <bits/stdc++.h>
using namespace std;
int main() {
  int h, w, k;
  cin >> h >> w >> k;
  int need = 0;
  for (int i = 1; i <= k; i++) {
    need += 2 * (w + h - 8 * (i - 1)) - 4;
  }
  cout << need;
}
