#073 - Easy

stack = []
operators = ['+','-','*','/']
do = ''

def get_input():
    print('\nStack =',stack, end='\n')
    stack.append(input("> "))

    try:
        k = int(stack[-1])

    except ValueError:
        if stack[-1] not in operators:
            print("'", stack.pop(), "' is not a valid input.", sep='')
        get_input()

while True:

    try:
        while stack[-1] not in operators:
                get_input()

    except IndexError:
        get_input()


    try:
        do = stack[-3] + ' ' + stack[-1] + ' ' + stack[-2]
        stack = stack[:-3]
        done = eval(do)
        print('\n',do, '=', done)
        stack.append(done)

        get_input()

    except IndexError:
        if stack[-1] in operators:
            print("Stack insufficient. '",stack.pop(),"' removed.", sep='')
            print(stack)
        get_input()