#include <bits/stdc++.h>
using namespace std;
bool isprime(long long n) {
  if (n < 2) return false;
  if (n == 2) return true;
  if (n % 2 == 0) return false;
  for (int i = 3; i * i <= n; i = i + 2) {
    if (n % i == 0) return false;
  }
  return true;
}
int main() {
  int w, h, k;
  cin >> w >> h >> k;
  int sum = 0;
  while (k--) {
    sum += (2 * w + (2 * (h - 2)));
    w -= 4;
    h -= 4;
  }
  cout << sum;
  return 0;
}
