#include <bits/stdc++.h>
using namespace std;
int n, m, k;
int main() {
  scanf("%d %d %d", &n, &m, &k);
  if (n < m) swap(n, m);
  if (m < k * 2 - 1) {
    printf("0\n");
    return 0;
  }
  if (m == k * 2 - 1) {
    printf("%d\n", (n - 2 * k + 3) / 2);
    return 0;
  }
  int cs = 2 * ((n - 2 * k + 1) + (m - 2 * k + 1));
  printf("%d\n", (cs + 1) / 2);
  return 0;
}
