class Yukicoder
  def initialize
    remain = gets.to_i
    n = gets.to_i
    c = []

    n.times do
      c << gets.to_i
    end

    answer = 0

    loop do
      c = c.sort.reverse
      max_val = c.max
      min_val = c.min
      a = c.inject(0){|sum, n| sum += (n - min_val)}/2
      b = c.inject(0){|sum, n| sum += (max_val-n)}

      if a < b || remain < b
        d = c.find{|i| i < max_val}
        if max_val - d <= 10
          c[0] -= 1
          answer += 1
          remain += 1
        else
          c[0] = d
          answer += (max_val-d)
          remain += (max_val-d)
        end
      else
        answer += b
        break
      end
    end

    puts answer
  end
end

Yukicoder.new