#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, k, n, m, x;
  vector<int> times;
  cin >> n >> m;
  cin >> x;
  while ((n > 0) && (m > 0)) {
    if ((n == 1) || (m == 1)) {
      times.push_back((n * m / 2) + 1);
    } else {
      k = ((n - 2) * 2 + m * 2) / 2;
      times.push_back(k);
    }
    n -= 2;
    m -= 2;
  }
  if (times.size() < x) {
    cout << 0 << endl;
  } else {
    cout << times[x - 1] << endl;
  }
  return 0;
}
