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