#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9 + 10;
int main() {
  ios::sync_with_stdio(false);
  srand(time(NULL));
  int w, h, k;
  cin >> w >> h >> k;
  int ans = 0;
  while (k--) {
    ans += 2 * w + 2 * h - 4;
    w -= 4;
    h -= 4;
  }
  cout << ans << endl;
  return 0;
}
