#include <bits/stdc++.h>
using namespace std;
int main() {
  int len, wide, num, ans = 0;
  cin >> len >> wide >> num;
  for (int i = 1; i <= num; i++) {
    ans += 2 * (len + wide) - 4;
    len -= 4;
    wide -= 4;
  }
  cout << ans;
}
