#include <bits/stdc++.h>
using namespace std;
int main(void) {
  long long n, m;
  long long x;
  int res = 0;
  cin >> n >> m >> x;
  n -= 2 * (x - 1);
  m -= 2 * (x - 1);
  if (n <= 0 || m <= 0) {
    cout << 0;
    return 0;
  }
  if (n == 1) res = m / 2 + (m & 0x01);
  if (m == 1)
    res = n / 2 + (n & 0x01);
  else
    res = n + m - 2;
  if (res > 0)
    cout << res;
  else
    cout << 0;
}
