我的python程序为什么出问题了

我今天学了python，一知半解，自己写了这个程序
但是运行全错误，不知道为什么，求解答！

# -*- coding: UTF-8 -*-
import time
import sys
print("你好，这是foromer编写的Math_bot")
time.sleep(2)
print("你将要进行数学运算吗？")
time.sleep(2)

y = "是"
n = "否"
a=input("是否继续：")   #用于回答上方问题

if a == y:
    print("那我们继续吧")
    time.sleep(1)
elif a == n:
    print("那就将结束了")
    time.sleep(2)
    sys.exit()

print("Math_bot包含五种运算，分别是：")
time.sleep(1.5)
print("加法")
time.sleep(0.5)
print("减法")
time.sleep(0.5)
print("乘法")
time.sleep(0.5)
print("除法")
time.sleep(0.5)
print("乘方")#告知运算
time.sleep(2)
choice = input("请选择运算：")#输入运算的选择

def add(x, y):
   return x + y
def subtract(x, y):
   return x - y
def multiply(x, y):
   return x * y
def divide(x, y):
   return x / y
def pow(x,y):
    return pow(x,y)

if choice == "加法":
    Num1 = input("请输入第一个加数：")
    time.sleep(1)
    Num2 = input("请输入第二个加数：")
    print(Num1,"+",Num2,"=", add(Num1,Num2))
elif choice == "减法":
    Num1 = input("请输入被减数：")
    time.sleep(1)
    Num2 = input("请输入减数：")
    print(Num1,"-",Num2,"=", subtract(Num1,Num2))
elif choice == "乘法":
    Num1 = input("请输入第一个乘数：")
    time.sleep(1)
    Num2 = input("请输入第二个乘数:")
    print(Num1,"*",Num2,"=", multiply(Num1,Num2))
elif choice == "除法":
    Num1 = input("请输入被除数：")
    time.sleep(1)
    Num2 = input("请输入除数：")
    print(Num1,"/",Num2,"=", divide(Num1,Num2))
elif choice == "乘法":
    Num1 = input("请输入底数：")
    time.sleep(1)
    Num2 = input("请输入指数：")
    print(Num1,"^",Num2,"=", pow(Num1,Num2))
time.sleep(2)
print("感谢用户的支持，我们已成功进行了一次数学运算")

input接收到的都是字符串需要转成数值 float()

错误信息为什么不发出来？