//area=(n*m)/ ((x+1)*(k-x+1))

//1: x==0;

//2: x=n-1

//3: x=m-1

# include <stdio.h>

long long max(long long x,long long y)

{

	return x>y?

x:y;

}

int main()

{

	long long  n,m,k,sum,t,ans;

	scanf("%lld%lld%lld",&n,&m,&k);

	sum=n+m-2;

	if(k>sum)

		printf("-1\n");

	else if(sum==k)

		printf("1\n");

	else

	{

		ans=0;

		if(n>=k+1)//x=0

		{

			ans=max(ans,m*(n/(k+1)));

		}

		else//x=n-1

		{

			t=k-n+2;

			ans=max(ans,m/t);

		}

		if(m>=k+1)//x=0

		{

			ans=max(ans,n*(m/(k+1)));

		}

		else//x=m-1

		{

			t=k-m+2;

			ans=max(ans,n/t);

		}

		printf("%lld\n",ans);

	}

	return 0;

}