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