#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  int n, m, x;
  cin >> n >> m >> x;
  int col = 0, odp = 0;
  while (n > 0 && m > 0) {
    ++col;
    int il;
    if (n == 1 || m == 1)
      il = (max(n, m) + 1) / 2;
    else
      il = n + m - 2;
    if (col == x) odp = il;
    n -= 2;
    m -= 2;
  }
  cout << odp;
  return 0;
}
