[Python] How to add a value to a whole list, but except specific values?

I wish to have a list of numbers, e.g. [3,6,-1,8,2,-1] and be able to add 2 to all elements except the -1s, for example, to get [5,8,-1,10,4,-1].

I've managed to add values to all elements, but have been unable to except values, which I think is due to me being unable to define specific elements within the code.

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