#include <cstdio>

using namespace std;
int cnt1,cnt2,n;

int main() {
    freopen("par.in", "r", stdin);
    freopen("par.out", "w", stdout);

    scanf("%d", &n);
    getchar();

    for (int i = 0; i < n; i++) {
        char c;
        scanf("%c", &c);
        if (c == ')') {
            if (cnt1==0) {
                cnt1++;
                cnt2++;
            }
            else {
                cnt1--;
            }
        } else {
            cnt1++;
        }
    }

    if (n%2==0)
        printf("%d", cnt2+cnt1/2);
    else
        printf("-1");
    return 0;
}