请编写一段PYTHON代码，功能要求：随机生成两个两位数及加减号（要保证减法算式的被减数大于减数），显示题目让学生输入计算结果，判断并显示结果的正确与否（或给予累积得分）。可考虑设置答题结束条件为时间限制（如1分钟内完成）或题目总数限制（如20题），也可以是其它条件（自行设置）。

import random
import time

score = 0
total_questions = 20
questions_answered = 0

start_time = time.time()

while questions_answered < total_questions and time.time() - start_time < 60:
num1 = random.randint(10, 99)
num2 = random.randint(10, 99)
operator = random.choice(['+', '-'])

if operator == '+':
answer = num1 + num2
question = f"{num1} + {num2} = "
else:
answer = num1 - num2
question = f"{num1} - {num2} = "

user_answer = input(question)

try:
user_answer = int(user_answer)
except ValueError:
user_answer = None

if user_answer == answer:
print("Correct!")
score += 1
else:
print("Incorrect.")

questions_answered += 1

print(f"You answered {score} out of {total_questions} questions correctly in {time.time() - start_time:.2f} seconds.")