import operator
import readline
from typing import Callable


class FifthStack:
    def __init__(self):
        self.stack: list[int] = []
        self.commands: dict[str, Callable] = {
            "push": self.push,
            "pop": self.pop,
            "swap": self.swap,
            "dup": self.dup,
        }
        self.binary_ops: dict[str, Callable[[int, int], int]] = {
            "+": operator.add,
            "-": operator.sub,
            "*": operator.mul,
            "/": operator.floordiv,
        }

    def _require(self, number, message):
        if len(self.stack) < number:
            print(f"ERROR: {message}")
            return False
        return True

    def _get_binary_op(self, command):
        return self.binary_ops.get(command)

    def _binary_op(self, operator):
        if not self._require(2, "two numbers required"):
            return
        b, a = self.stack.pop(), self.stack.pop()
        try:
            self.stack.append(operator(a, b))
        except ZeroDivisionError as e:
            print(f"ERROR: division by zero")
            self.stack.extend([a, b])

    def push(self, value):
        try:
            self.stack.append(int(value))
        except ValueError:
            print("ERROR: integer required")

    def pop(self):
        if not self.stack:
            print("ERROR: stack empty")
            return
        self.stack.pop()

    def swap(self):
        if not self._require(2, "two numbers required"):
            return
        self.stack[-1], self.stack[-2] = self.stack[-2], self.stack[-1]

    def dup(self):
        if self._require(1, "stack empty"):
            self.stack.append(self.stack[-1])

    def execute(self, command: str):
        tokens = command.lower().strip().split()
        if not tokens:
            return

        command, argument = tokens[0], tokens[1] if len(tokens) > 1 else None
        function = self.commands.get(command)

        if function:
            if command == "push":
                function(argument)
            else:
                function()
        elif command in self.binary_ops:
            self._binary_op(self._get_binary_op(command))
        else:
            print("ERROR: unknown command")


def main():
    fifth = FifthStack()

    while True:
        print(f"stack is {fifth.stack}")
        try:
            if (command := input().strip().lower()) == "exit":
                break
            if command:
                fifth.execute(command)
        except (EOFError, KeyboardInterrupt):
            break


if __name__ == "__main__":
    main()