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