class Stack:

    class StackNode:

        def __init__(self, value):
            self.value = value
            self.next = None


    def __init__(self):
        self.head = None


    def add(self, value):
        if self.head is None:
            self.head = self.StackNode(value)
            return

        current = self.head
        while current.next is not None:
            current = current.next

        current.next = self.StackNode(value)


    def remove(self, value):
        if self.head is None:
            return

        self.head = self.head.next


    def peek(self):
        return self.head


    def is_empty(self):
        return self.head is None