#include <bits/stdc++.h>
int main() {
  int n, m, x;
  int t, r(0);
  std::scanf("%d%d%d", &n, &m, &x);
  for (int i(0); i < n; ++i) {
    for (int j(0); j < m; ++j) {
      if ((i + j) % 2 != 0) continue;
      t = std::min({i + 1, j + 1, n - i, m - j});
      r += t == x;
    }
  }
  std::printf("%d\n", r);
  return 0;
}
