计算公式有问题 elec_num = 90 的话是-41.053 如何解决，小于100 不可取

因为elec_num这个数值就定义要比400大，如果elec_num要随意输出的话，估计要用到判断语句，还没学到哪里去

print("Welcome to ladder electricity calculator")    # 欢迎使用阶梯电费计算器
'''
一档 0=<电量<=240，电价0.4883
二档 240<电量<=400 ,电价0.5383
三档 电量>400  ,电价0.7883
'''

e_num = int(input("please enter your electricity consumption:"))    # 输入当月用电量
if 0 < e_num <= 240:
   e_charge = 240*0.4883
   print("The electricity bill your should pay for the month is:{}".format(e_charge))  # 当月应该缴纳的电费为
elif 240 < e_num <= 400:
   e_charge = 240*0.4883+(400-240)*0.5383
   print("The electricity bill your should pay for the month is:{}".format(e_charge))  # 当月应该缴纳的电费为
elif e_num >400:
   e_charge = 240 * 0.4883 + (400 - 240) * 0.5383 + (e_num - 400) * 0.7883
   print("The electricity bill your should pay for the month is:{}".format(e_charge))  # 当月应该缴纳的电费为