n,k = gets.chomp.split(" ").map(&:to_i)
s = gets.chomp
stack = []
x = 1
a = -1
s.each_char do |c|
  if(c == "(")
    stack.push(x) 
  else
    a = stack.pop
  end
  break if(x == k)
  x += 1
end
x = n
stack = []
b = -1
s.reverse.each_char do |c|
  if(c == ")")
    stack.push(x) 
  else
    b = stack.pop
  end
  break if(x == (k))
  x -= 1
end
puts [a,b].max