#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e18;
const double eps = 1e-6;
int main() {
  long long w, h, k;
  cin >> w >> h >> k;
  long long ans = 0;
  for (long long i = 1; i <= k; i++) {
    ans += max((long long)1, (((w - 4 * (i - 1)) + (h - 4 * (i - 1)) - 2)) * 2);
  }
  cout << ans;
  return 0;
}
