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