#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long w, h, k, ans = 0;
  cin >> w >> h >> k;
  for (long long i = h, j = w; i > 0, j > 0, k > 0; i -= 4, j -= 4) {
    ans += 2 * i + 2 * j;
    ans -= 4;
    k--;
  }
  cout << ans << endl;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long double t = 1;
  while (t--) {
    solve();
  }
}
