#include<iostream>
#include<stdio.h>
#define ll long long
using namespace std;
ll n, m, k;
ll get(ll a)
{
	return (n / (a + 1)) * (m / (k - a + 1));
}
int main()
{
	cin >> n >> m >> k;
	if(n + m - 2 < k)
		cout << -1;
	ll l, r;
	l = max(0ll, k + 1 - m), r = min(k, n - 1);
	cout << max(get(l), get(r)) << endl;
	return 0;
}