import sys
input = sys.stdin.readline

N=int(input())
S=input().strip()
R,M=map(int,input().split())

if N%2!=0:
    print(-1)
    exit()


x=0
y=0

for s in S:
    if s=="(":
        x+=1
    else:
        y+=1

ANS=(max(x,y)-min(x,y))//2

print(ANS)