#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, k, s = 0;
  cin >> r >> c >> k;
  while (k != 0) {
    k--;
    s = s + 2 * r;
    s = s + 2 * (c - 2);
    r = r - 4;
    c = c - 4;
  }
  cout << s << endl;
  return 0;
}
