#include <bits/stdc++.h>
int gild_rng(int a, int b, int c) {
  int sum = 0;
  for (int n = 0; n < c; n++) {
    int ans;
    ans = 2 * (a + b - (8 * n)) - 4;
    sum = sum + ans;
  }
  return sum;
}
int main() {
  int w, h, k;
  scanf("%d %d %d", &w, &h, &k);
  int ring = gild_rng(w, h, k);
  printf("%d", ring);
  return 0;
}
