#include <bits/stdc++.h>
using namespace std;
signed main() {
  long long w, h, k, ans = 0;
  cin >> w >> h >> k;
  while (k-- && w >= 2 && h >= 2) {
    ans += 2 * (max(w, h)) + 2 * (min(w, h) - 2);
    w -= 4;
    h -= 4;
  }
  cout << ans << endl;
}
