#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  int w, h, k, maxi, mini, sum = 0;
  cin >> w >> h >> k;
  maxi = max(w, h);
  mini = min(w, h);
  for (int i = 0; i < k; i++) {
    sum += (maxi - 4 * i) * 2 + 2 * (mini - 4 * i - 2);
  }
  cout << sum;
}
