class Cal
  attr_accessor :num
  def initialize
      @num = 0
  end
  def add
      @num += 1
  end
end
 
cal = Cal.new
100000.times { cal.add }
100000.times { cal.add }
puts "#{cal.num}"  # => 200000
 
cal.num= 0
thread1 = Thread.new { 100000.times { cal.add } }
thread2 = Thread.new { 100000.times { cal.add } }
 
puts "#{cal.num}"  # => 110541 ??