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