#include <bits/stdc++.h>
using namespace std;
int main() {
  int w, h, k, sum = 0;
  cin >> w >> h >> k;
  for (int i = 1; i <= k; ++i) {
    int nw = w - 4 * (i - 1);
    int nh = h - 4 * (i - 1);
    if (nw > 0 and nh > 0)
      sum += (2 * nw + 2 * (nh - 2));
    else
      break;
  }
  cout << sum << endl;
  return 0;
}
