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