#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++) {
    if ((w - 2) >= 0 && (h - 2) >= 0) {
      sum = sum + w * h - ((w - 2) * (h - 2));
      w = w - 4;
      h = h - 4;
    }
  }
  printf("%d", sum);
}
