#include <bits/stdc++.h>
using namespace std;
int n, m, x, t;
int main() {
  cin >> n >> m >> x;
  t = 1;
  while (n > 0 && m > 0) {
    if (t == x) {
      if (n == 1 && m == 1)
        cout << 1 << endl;
      else if (n == 1 || m == 1)
        cout << max(n + 1, m + 1) / 2 << endl;
      else
        cout << (n + n + m + m - 4) / 2 << endl;
      return 0;
    }
    t++;
    n -= 2;
    m -= 2;
  }
  cout << 0 << endl;
  return 0;
}
