I am a beginner in python programming and I encountered an issue with my code.

When a user types an invalid operation, it notifies the users but exits the program (line 33). How can I get it to ask user to enter a math operation again?

```
#This python program calculates the sum, difference, product, or quotient of two numbers defined by users.

#Define add function and return the result of num1 + num2
def add(num1, num2):
    return num1 + num2

#Define subract function and return the result of subtracting num1 - num2
def sub(num1, num2):
    return num1 - num2

#Define multiplication function and return the result of multiplying num1 * num2
def mul(num1, num2):
    return num1 * num2

#Define division function and return the result of dividing num1 / num2
def div(num1, num2):
    return num1 / num2

#Define main purpose/function of the program
def main():

    #Ask what math operation to perform
    operation = input("What do you want to do? (+, -, *, /): ")

    #If the operation is not +, -, *, or /, invalid operation
    if(operation != '+' and operation != '-' and operation != '*' and operation != '/'):
        print("You must enter a valid operation!")

    #If valid, perform specified operation
    else:
        var1 = int(input("Enter num1: "))
        var2 = int(input("Enter num2: "))
        if(operation == '+'):
            print(add(var1, var2))
        elif(operation == '/'):
            print(div(var1, var2))
        elif(operation == '-'):
            print(sub(var1, var2))
        else:
            print(mul(var1, var2))

main()

```