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