#include <bits/stdc++.h>
using namespace std;
const int MOD = 1000000007;
const double eps = 1e-4;
int n, m, x;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  if (fopen("test.in", "r")) {
    freopen("test.in", "r", stdin);
  }
  cin >> n >> m >> x;
  if (n - 2 * x + 2 <= 0 || m - 2 * x + 2 <= 0) {
    cout << 0;
  } else {
    cout << max(n + m - 4 * x + 2, 1);
  }
  return 0;
}
