def inputcheck(n):
    while True:
        try:
            return float(n)
        except:
            print("输入错误,输入值只能为数字:")
            n = input()

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 main():
    print("Select operation.", "1.Add", "2.Subtract", "3.Multiply", "4.Divide")
    
    choice = input("Enter choice(1/2/3/4):")
    num11 = input("Enter first number: ")
    num1 = inputcheck(num11)
    num22 = input("Enter second number: ")
    num2 = inputcheck(num22)
    if choice == '1':
        print(num1, "+", num2, "=", add(num1, num2))
    elif choice == '2':
        print(num1, "-", num2, "=", subtract(num1, num2))
    elif choice == '3':
        print(num1, "*", num2, "=", multiply(num1, num2))
    elif choice == '4':
        if num2 == 0:
            print("division by zero !")
        else :
            print(num1, "/", num2, "=", divide(num1, num2))
    else :
            print("Invalid input")

if __name__ == '__main__':
    main()