#include <bits/stdc++.h>
long count(long n, long m) {
  if (n < 1 || m < 1) return 0;
  if (n == 1) return (m + 1) / 2;
  if (m == 1) return (n + 1) / 2;
  return 2 * (n / 2 + m / 2) - !(n & 1) - !(m & 1);
}
int main() {
  long n, m, k;
  scanf("%ld%ld%ld", &n, &m, &k);
  printf("%ld\n", count(n - (k - 1) * 2, m - (k - 1) * 2));
  return 0;
}
