#include <bits/stdc++.h>
using namespace std;
long long n, m, x;
void solve() {
  cin >> n >> m >> x;
  n = max(0LL, n - 2 * (x - 1));
  m = max(0LL, m - 2 * (x - 1));
  long long n1 = max(0LL, n - 2);
  long long m1 = max(0LL, m - 2);
  long long ans = n * m / 2 + n * m % 2 - n1 * m1 / 2 - n1 * m1 % 2;
  cout << ans << "\n";
}
int main() {
  solve();
  return 0;
}
