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