class IntegerStack:
    def __init__(self):
        self.stack = [] * 128

    def depth(self):
        return len(self.stack)

    def tos(self):
        return self.stack[-1]

    def push(self, v):
        self.stack.append(v)

    def append(self, v):
        self.stack.append(v)

    def pop(self):
        return self.stack.pop()

    def dup(self):
        self.stack.append(self.stack[-1])

    def drop(self):
        self.stack.pop()

    def swap(self):
        a = self.stack[-2]
        self.stack[-2] = self.stack[-1]
        self.stack[-1] = a