#include <bits/stdc++.h>
const int inf = 2147483640;
using namespace std;
int main() {
  long long n, m, x;
  cin >> n >> m >> x;
  long long tn = n - 2 * (x - 1);
  long long tm = m - 2 * (x - 1);
  if ((tn <= 0) || (tm <= 0)) {
    cout << 0;
    return 0;
  }
  if (tn == 1) {
    cout << tm / 2 + tm % 2;
    return 0;
  } else if (tm == 1) {
    cout << tn / 2 + tn % 2;
    return 0;
  }
  cout << (2 * tn + 2 * tm - 4) / 2;
}
