#include <bits/stdc++.h>
int main() {
  int m, n, k, count;
  scanf("%d %d %d", &m, &n, &k);
  count = m * 2 + (n - 2) * 2;
  for (int i = 1; i < k; i++) {
    m -= 4;
    n -= 4;
    count += m * 2 + (n - 2) * 2;
  }
  printf("%d\n", count);
}
