#include <bits/stdc++.h>
using namespace std;
const double pi = 3.14159265359;
const double eps = 1e-9;
const int mod = 1e9 + 7;
const int mod1 = 1e9 + 9;
const int INF = 2e9;
const long long INFLL = 4e18;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, m, k;
  cin >> n >> m >> k;
  int ans = 0;
  for (int i = 1, j = 1, cnt = 0;
       i <= (n + 1) / 2 && j <= (m + 1) / 2 && cnt < k; i += 2, j += 2, cnt++) {
    ans += (n - (i - 1) * 2) * 2 + (m - (j - 1) * 2) * 2 - 4;
  }
  cout << ans;
  return 0;
}
