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