#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n, m;
  cin >> n >> m;
  int x;
  cin >> x;
  x--;
  n -= 2 * x;
  m -= 2 * x;
  if (n < 0 || m < 0)
    cout << 0;
  else if (n == 1 && m == 1)
    cout << 1;
  else
    cout << (n + m) - 2;
  return;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
