Solving Roman Numerals

I'm very new to coding, just starting to pick it up this week actually, but I'm wondering if there is a way to return the value for the key with the highest int(value) not greater than number input. I structured my dictionary reverse to what the help suggests (i.e (4, 'IV')). So I am trying to get the code to iterate through the dictionary and find the highest value key not greater than the input and put it in my variable string. I apologize if this is confusing or if it something not feasible for this project.