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