#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(0);
  long long int a, b, c, d, n, t, count = 0, sum = 0, result, i, j, k;
  bool is;
  string s;
  cin >> a >> b >> n;
  for (i = 0; i < n; i++) {
    sum += (2 * a) + (2 * b) - 4;
    a -= 4;
    b -= 4;
  }
  cout << sum;
  return 0;
}
