#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, x;
  cin >> n >> m >> x;
  if (n + 1 - 2 * (x - 1) > 0 && m + 1 - 2 * (x - 1) > 0)
    cout << max(1, ((n - 2 * (x - 1)) + (m - 2 * (x - 1))) - 2);
  else
    cout << 0;
}
