#include <bits/stdc++.h>
using namespace std;
struct node {
  int value;
  node* right;
  node* left;
  node* midel;
};
int main() {
  int w, h, k;
  cin >> w >> h >> k;
  int count = 0;
  int i = 0;
  while (k > 0) {
    count += 2 * (w - (i * 4) + h - (i * 4)) - 4;
    i++;
    k--;
  }
  cout << count;
  return 0;
}
