#include <bits/stdc++.h>
using namespace std;
const int MAXN = -1;
int n, m, k;
int main() {
  while (cin >> n >> m >> k) {
    while (--k) {
      n -= 2;
      m -= 2;
    }
    if ((n > 0) && (m > 0))
      printf(
          "%d\n",
          ((min(n, m) > 0) ? (n * m + 1) / 2 : 0) -
              ((min((n - 2), (m - 2)) > 0) ? ((n - 2) * (m - 2) + 1) / 2 : 0));
    else
      printf("0\n");
  }
  return 0;
}
