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