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