#include <bits/stdc++.h>
using namespace std;
long long n, m, k;
int main() {
  cin >> n >> m >> k;
  if (k > n + m - 2) {
    cout << -1 << endl;
    return 0;
  }
  long long ans1, ans2;
  if (k >= m) {
    ans1 = n / (k - m + 2);
  } else {
    ans1 = (m / (k + 1)) * n;
  }
  if (k >= n) {
    ans2 = m / (k - n + 2);
  } else {
    ans2 = (n / (k + 1)) * m;
  }
  cout << max(ans1, ans2) << endl;
  return 0;
}
