#include <bits/stdc++.h>
using namespace std;
int main() {
  int h, w, s, k, a, b;
  cin >> w >> h >> k;
  int i = 0;
  s = 0;
  while (i < k) {
    ++i;
    a = h - 4 * (i - 1);
    b = w - 4 * (i - 1);
    s = s + 2 * a + 2 * b - 4;
  }
  cout << s;
}
