#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, n, ans = 0;
  cin >> a >> b >> n;
  for (int i = 0; i < n; i++) {
    ans += (a + b) * 2 - 4;
    a -= 4;
    b -= 4;
  }
  cout << ans << endl;
  return 0;
}
