Order total

Hello

I have looked around and haven't found a solution for my (I think) easy problem.
I'm not much of a developper so it explains why I'm stuck

I'm creating an easy online order form to well... order a product

Price is fixed (1555 €)
user has 1 questions (radio buttons) with 3 options

Q1
R1= 0 €
R2 = 75 €
R3 : 25 €

I'd like to have an order total showing somewhere in my form that add's up to the 1555 € when respons 1 or 2 is selected

Any help is appreciated

Thank's