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