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