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