while (line = gets) do
  N, M = line.split.map(&:to_i)
  break if N==0 && M==

  current_state = 1
  rules = Array.new(N) { |i| i = 0 }
  actions = Array.new(M) { |i| i = 0 }

  N.times do |i|
    rules[i] = gets.to_i
  end

  M.times do |i|
    actions[i] = gets.to_i
  end

  actions.each_with_index do |action, i|
    #puts "-----------#{i}-------------------------------"
    #puts "current_state : #{current_state}"
    #puts "action: #{action}"
    current_state += action
    if current_state >= 10
      p i+1
      break
    end
    current_state += rules[current_state-1]
    if current_state >= 10
      p i+1
      break
    end
  end
end