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