class Stack:
    def __init__(self):
        self._items = []

    def push(self, item):
        """Adds an item to the top of the stack."""
        self._items.append(item)

    def pop(self):
        """Removes and returns the item from the top of the stack."""
        if not self.is_empty():
            return self._items.pop()
        raise IndexError("pop from empty stack")

    def peek(self):
        """Returns the item at the top of the stack without removing it."""
        if not self.is_empty():
            return self._items[-1]
        raise IndexError("peek from empty stack")

    def is_empty(self):
        """Checks if the stack is empty."""
        return len(self._items) == 0

    def size(self):
        """Returns the number of items in the stack."""
        return len(self._items)

# Example Usage:
# my_stack = Stack()
# my_stack.push(10)
# my_stack.push(20)
# print(my_stack.peek()) # 20
# print(my_stack.pop())  # 20
# print(my_stack.is_empty()) # False
# my_stack.pop()
# print(my_stack.is_empty()) # True