#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;   }   