N, K = map(int, input().split())
S = input()

ans = -1
stack = []
for i in range(N):
    if S[i] == '(':
        stack.append(i)
    elif S[i] == ')':
        counterpart = stack.pop()
        if i+1 == K:
            ans = counterpart +1
            break
        elif counterpart + 1 == K:
            ans = i+1
            break
print(ans)