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