#include <bits/stdc++.h>
int count(int x, int y) {
  int r;
  r = 2 * (x + y - 2);
  return r;
}
int main() {
  int w, h, k, i, s;
  scanf("%d", &w);
  scanf("%d", &h);
  scanf("%d", &k);
  s = 0;
  i = 0;
  while (i < k) {
    s = s + count(w - 4 * i, h - 4 * i);
    i = i + 1;
  }
  printf("%d", s);
  return 0;
}
