class Stack
    def initialize
        @data = []
    end

    def push(x)
        @data.push(x)
    end

    def pop
        @data.pop
    end

    def top
        @data[@data.length - 1]
    end
end

s = Stack.new
s.push(1)
s.push(2)
s.push(3)
s.push(4)
s.push(10)
puts(s.pop) #=> 10
puts(s.top) #=> 4