#include <bits/stdc++.h>
int main() {
  int w, h, k, area;
  while (scanf("%d%d%d", &w, &h, &k) == 3) {
    area = (w + h - 2) * 2;
    for (int i = 2; i <= k; i++) {
      w -= 4;
      h -= 4;
      area += (w + h - 2) * 2;
    }
    printf("%d\n", area);
  }
  return 0;
}
