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