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