#include <bits/stdc++.h>
using namespace std;
using ll = long long;
void solve() {
  int w, h, k;
  cin >> w >> h >> k;
  ll ans = 0;
  while (w >= 1 && h >= 1 && k > 0) {
    ans += 2 * w + 2 * (h - 2);
    h -= 4;
    w -= 4;
    k--;
  }
  cout << ans << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
}
