#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 100;
const double PI = 3.14159;
long long n, m, k, a[N], c, t, ans;
string s, x;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cin >> n >> m >> k;
  while (k--) {
    ans += n * 2;
    ans += (m - 2) * 2;
    m -= 4;
    n -= 4;
  }
  cout << ans << endl;
  return 0;
}
