#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, m, x;
  cin >> n >> m >> x;
  if (min(m, n) - 2 * (x - 1) < 0) {
    printf("0\n");
    return 0;
  }
  n -= 2 * (x - 1);
  m -= 2 * (x - 1);
  if (n == 1 || m == 1) {
    printf("%d\n", (int)(n * m + 1) / 2);
    return 0;
  }
  int ans = 2 * (n + m) - 4;
  ans /= 2;
  printf("%d\n", ans);
  return 0;
}
