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