MIT OCW 6.0001 pset 1b，计算所需的保存月数3个测试，第一个是正确的，另外两个比我的代码给出的多1个月，为什么？

问题描述

#Code
'''

annual_salary = float(input('What is your starting annual salary?'))
monthly_salary = annual_salary/12
portion_saved = float(input('What percentage of your salary will you save each month(in decimals)?'))
Total_cost = float(input('How much is your dream home?'))
semi_annual_raise = float(input('What is your raise every 6 months(in decimals)?'))
portion_down_payment = 0.25*Total_cost
r = 0.04
current_savings = 0
months = 0
while current_savings < portion_down_payment:
    months += 1
    if months % 6 == 0:
        monthly_salary += semi_annual_raise*monthly_salary
    current_savings += portion_saved*monthly_salary + current_savings*r/12
print('Number of months:',months)

＃测试 '''

      1:Enter your starting annual salary:  120000
        Enter the percent of your salary to save,as a decimal:  .05 
        Enter the cost of     your dream home:  500000
        Enter the semi­annual raise,as a decimal:  .03
        Number of months:  142

      2:Enter your starting annual salary:  80000
        Enter the percent of your salary to save,as a decimal:  .1 
        Enter the cost of your dream home:  800000
        Enter the semi­annual raise,as a decimal:  .03
        Number of months:  159

      3:Enter your starting annual salary:  75000
        Enter the percent of your salary to save,as a decimal:  .05 
        Enter the cost of your dream home:  1500000
        Enter the semi­annual raise,as a decimal:  .05
        Number of months:  261

这是3个测试，请帮助 最后2个分别给我的代码结果158和260 它是一种算法，可以根据工资，半年工资，每月储蓄额和投资回报率来计算节省房屋首付所需的月数。

解决方法

#Code

annual_salary = float(input('What is your starting annual salary?'))
monthly_salary = annual_salary/12
portion_saved = float(input('What percentage of your salary will you save each month(in decimals)?'))
Total_cost = float(input('How much is your dream home?'))
semi_annual_raise = float(input('What is your raise every 6 months(in decimals)?'))
portion_down_payment = 0.25*Total_cost
r = 0.04
current_savings = 0
months = 0
while current_savings < portion_down_payment:
    current_savings += portion_saved*monthly_salary + current_savings*r/12
    months += 1
    if months % 6 == 0:
        monthly_salary += semi_annual_raise*monthly_salary
print('Number of months:',months)

＃months = 1的错位导致了错误