#include <bits/stdc++.h>
using namespace std;
int main() {
  istream &is = cin;
  ostream &os = cout;
  int n, m, x;
  cin >> n >> m >> x;
  if (n < m) swap(n, m);
  int res;
  if (m < 2 * x - 1)
    res = 0;
  else if (m == 2 * x - 1 && n == 2 * x - 1)
    res = 1;
  else if (m == 2 * x - 1) {
    int k = n - 2 * x;
    if (k % 2 == 0)
      res = k / 2;
    else
      res = k / 2 + 1;
  } else {
    res = m + n - 4 * x + 2;
  }
  cout << res << endl;
  return 0;
}
