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