#include <bits/stdc++.h>
int main() {
  int w, h, k, s = 0;
  scanf("%d %d %d", &w, &h, &k);
  while (k > 0) {
    s += 2 * (w + h - 2);
    w -= 4;
    h -= 4;
    k--;
  }
  printf("%d\n", s);
  return 0;
}
