day_4.23 Simple calculator

''' 
Simple addition, subtraction, multiplication and division calculator demo 
2018-4-23 19:32:49 
''' 
#1. Interface 

print("="*50) 

print("Welcome to calculator v0.1") 
print(" 1. Use addition") 
print(" 2. Use subtraction") 
print(" 3. Use multiplication") 
print(" 4. Use division") 
print(" 5. Exit the system") 

print("="*50) 
#2. Code logic 
while True: 

    num = int(input("Please enter a number:")) 
    #3. Function implementation 
        #Addition has parameter function, 
    def plus(a,b): 
        print("Simple function can only input two Add numbers") 
        result =a+b 
        print("%d+%d=%d"%(a,b,result)) 
    if num==1: 
        num1 = int(input("Enter the first number: ")) 
        num2 = int(input("Enter the second number:")) 
        plus(num1,num2) 
    #subtraction 
    elif num==2:
        pass 
    #Multiplication 
    elif num==3: 
         pass 
    #Division 
    elif num==4: 
        pass 
    elif num==5: 
        print("Exit the system!") 
        break 
    else: 
        print("Your input is incorrect, please re-enter") 

    print ("")