#include <fstream>
#include <stack>
using namespace std;
ifstream f("par.in");
ofstream g("par.out");
stack <int> stiva;
int n;
char c;
int main()
{
	f >> n;
	if (n % 2 == 1)
	{
		g << -1;
		return 0;
	}
	int nr = 0;
	while (f >> c)
	{
		if (c == '(')
			stiva.push(1);
		else
		{
			if (!stiva.empty())
				stiva.pop();
			else
			{
				nr++;
				stiva.push(1);
			}
		}
	}
	nr = nr + stiva.size() / 2;
		g << nr;
}