#include <bits/stdc++.h>
using namespace std;
int64_t max(int32_t a, int64_t b) { return max((int64_t)a, b); }
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  pair<int64_t, int64_t> lc, rc;
  cin >> rc.second >> rc.first;
  lc.first = lc.second = 1;
  int64_t k;
  cin >> k;
  int64_t cnt = 0;
  int64_t ans = 0;
  int64_t cur = 1;
  while (cnt < k) {
    ans += (rc.first - lc.first + 1) * (rc.second - lc.second + 1);
    rc.first--;
    rc.second--;
    lc.second++;
    lc.first++;
    ans -= (rc.first - lc.first + 1) * (rc.second - lc.second + 1);
    cnt++;
    rc.first--;
    rc.second--;
    lc.second++;
    lc.first++;
  }
  cout << ans;
}
