#include <bits/stdc++.h>
using namespace std;
int f(int n, int m, int x) {
  x *= 2;
  if (x > min(n, m)) return 0;
  return ((n - x) * (m - x) + 1) / 2;
}
int main() {
  int n, m, x;
  scanf("%d %d %d", &n, &m, &x);
  if (2 * (x - 1) >= min(n, m))
    printf("0\n");
  else
    printf("%d\n", f(n, m, x - 1) - f(n, m, x));
  return 0;
}
