#include <bits/stdc++.h>
using namespace std;
int N, M, x, rs;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cin >> N >> M >> x;
  N -= 2 * (x - 1);
  M -= 2 * (x - 1);
  if (N <= 0 || M <= 0) return cout << 0, 0;
  if (N == 1 && M == 1) return cout << 1, 0;
  rs += N + M - 2;
  cout << rs;
  return 0;
}
