#include <bits/stdc++.h>
#pragma warning(disable : 4996)
using namespace std;
int main() {
  int h, w, k, count = 0;
  scanf("%d %d %d", &h, &w, &k);
  for (size_t i = 0; i < k; i++) {
    if (h == 1 && w == 1) {
      count++;
    } else if (h == 1) {
      count += w;
    } else if (w == 1) {
      count += h;
    } else {
      count += (h * 2) + (w - 2) * 2;
    }
    h -= 4;
    w -= 4;
  }
  printf("%d\n", count);
  return 0;
}
