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