import string

def is_valid_integer(numstr):
    i = 0
    is_correct = True

    while i < len(numstr) and is_correct:
        if numstr[i] not in string.digits:
            is_correct = False
        i += 1

    return is_correct

def is_valid_operator(operatorstr):
    return len(operatorstr) == 1 and operatorstr in "+-*/"

def input_sth(prompt, fail_msg, validator, chances=1):
    is_correct = False
    while chances > 0 and not is_correct:
        input_value = input(prompt)
        if validator(input_value):
            is_correct = True
        else:
            print (fail_msg)
            chances -= 1

    return input_value, is_correct

def calculate(operand1, operand2, operator):
    result = ''
    if operator == '+':
        result = operand1 + operand2
    elif operator == '*':
        result = operand1 * operand2
    elif operator == '-':
        result = operand1 - operand2
    elif operator == '/':
        result = operand1 / operand2
    else:
        result = "Error: invalid operator!"

    return result

everything_is_ok = True
while everything_is_ok:
    first_number, everything_is_ok = input_sth("Enter a number: ", "Critical error: Incorrect number!", is_valid_integer)

    if everything_is_ok:
        operator, everything_is_ok = input_sth("Enter a sign: ", "Ivalid sign", is_valid_operator, 3)

    if everything_is_ok:
        second_number, everything_is_ok = input_sth("Enter another number: ",
            "Critical error: Incorrect number!", is_valid_integer)

    if everything_is_ok:
        result = calculate(int(first_number), int(second_number), operator)
        print("Result:", result)