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