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