#include <bits/stdc++.h>
using namespace std;
int main() {
  int w, i, h, k, s;
  cin >> w >> h >> k;
  s = w * 2 + (h - 2) * 2;
  if (k > 1) {
    for (i = 2; i <= k; i++) {
      h = h - 4;
      w = w - 4;
      s = s + w * 2 + (h - 2) * 2;
    }
  }
  cout << s;
}
