module Exchange
  def solve(c, y)
    cex = y / 500 * 5
    ch = y % 500 / 100
    if ch >= c
      "no exchange"
    else
      nd = ((c - ch) + 4) / 5 * 5
      if nd <= cex
        ch + nd
      else
        "can't exchange"
      end
    end
  end
  module_function :solve
end

c, y = gets.chomp.split(/\s+/).map(&:to_i)
  
puts Exchange.solve(c, y)