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