#include <bits/stdc++.h>
int main() {
  int n, m, x, ans;
  scanf("%d%d%d", &n, &m, &x);
  ans = 0;
  for (int i = x; i <= n - x + 1; i++)
    for (int j = x; j <= m - x + 1; j++)
      if (((i + j) & 1) == 0 &&
          (i == x || i == n - x + 1 || j == x || j == m - x + 1))
        ans++;
  printf("%d\n", ans);
  return 0;
}
