import operator
import readline

class FifthStack:
    def __init__(self):
        self.stack = []
        self.commands = {
            'push': self.push,
            'pop': self.pop,
            'swap': self.swap,
            'dup': self.dup,
            '+': self.add,
            '-': self.sub,
            '*': self.mul,
            '/': self.div,
        }
        self.binary_ops = {
            '+': operator.add,
            '-': operator.sub,
            '*': operator.mul,
            '/': operator.floordiv,
        }
    
    def _require(self, number, message):
        if len(self.stack) < number:
            print(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
        a, b = self.stack.pop(), self.stack.pop()
        try:
            self.stack.append(operator(a, b))
        except Exception as e:
            print(f"ERROR: {e}")
            self.stack.extend([a, b])
    
    def push(self, value):
        try:
            self.stack.append(int(value))
        except ValueError:
            print("ERROR: integer required")

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

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

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

    def add(self, _value):
        self._binary_op(self._get_binary_op('+'))

    def sub(self, _value):
        self._binary_op(self._get_binary_op('-'))

    def mul(self, _value):
        self._binary_op(self._get_binary_op('*'))

    def div(self, _value):
        self._binary_op(self._get_binary_op('/'))

    def execute(self, command):
        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)

        try:
            if function: function(argument)
            else: print("ERROR: unknown command")
        except:
            print("ERROR: invalid command")

def main():
    fifth = FifthStack()
    
    while True:
        print(f'stack is: {fifth.stack}')
        command = input()
        if command.lower() == 'exit':
            break
        fifth.execute(command)

if __name__ == '__main__':
    main()