#include <bits/stdc++.h>
using namespace std;
int n, m, k;
long long ans;
int main() {
  cin >> n >> m >> k;
  if (k > n + m - 2) {
    cout << "-1" << endl;
    return 0;
  }
  if (n >= m) swap(n, m);
  if (k <= m - 1) {
    ans = (long long)(m / (k + 1)) * n;
  } else {
    ans = (long long)(n / (k - m + 2));
  }
  if (k <= n - 1) {
    ans = max(ans, (long long)(n / (k + 1)) * m);
  } else {
    ans = max(ans, (long long)(m / (k - n + 2)));
  }
  cout << ans << endl;
  return 0;
}
