#include <bits/stdc++.h>
int main() {
  int a, b, c, i, sum = 0;
  scanf("%d %d %d", &a, &b, &c);
  for (i = 0; i < c; i++) {
    sum = (2 * a) + (2 * (b - 2)) + sum;
    a = a - 4;
    b = b - 4;
  }
  printf("%d", sum);
  return 0;
}
