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