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