#include <bits/stdc++.h>
using namespace std;
int main() {
  unsigned int n, m, l;
  cin >> n >> m >> l;
  int s = 0;
  while (l--) {
    s += 2 * (n + m - 2);
    n -= 4;
    m -= 4;
  }
  cout << s;
}
