#include <bits/stdc++.h>
using namespace std;
int n, m, x;
vector<int> v;
int main() {
  cin >> n >> m >> x;
  if (x > n / 2 + 1)
    cout << 0;
  else {
    for (int i = 2; i <= x; i++) n -= 2, m -= 2;
    if (n == 1 && m == 1) {
      cout << 1;
      return 0;
    }
    if (n <= 0 || m <= 0) {
      cout << 0;
      return 0;
    }
    int tx1 = n / 2 + n % 2, tx2;
    tx2 = tx1;
    if (m % 2 == 0 && n % 2 == 1) tx2--;
    int ty1, ty2;
    ty1 = m / 2 + m % 2 - 1;
    if (m % 2 == 1) ty1--;
    if (n % 2 == 1)
      ty2 = m / 2 + m % 2;
    else
      ty2 = m / 2 + m % 2 - 1;
    if (n % 2 == 1) ty2--;
    if (m % 2 == 1 && n % 2 == 1) ty2--;
    cout << tx1 + tx2 + ty1 + ty2;
  }
}
