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