#include <bits/stdc++.h>
using namespace std;
template <class T>
int len(const T& c) {
  return (int)c.size();
}
int n, m, x, ans;
int main() {
  cin >> n >> m >> x;
  for (int i(0), _n(x); i < _n; ++i) {
    if (n <= 0 || m <= 0) {
      ans = 0;
      break;
    }
    ans = (2 * n + max(0, 2 * (m - 2)) + 1) / 2;
    n -= 2;
    m -= 2;
  }
  cout << ans;
}
