If your swimsuit forms an “X” on the back with the straps, this means you have four quadrants upon which to slather sunscreen: top, bottom, left, and right.
If you have a friend present, it is not completely out of the question to ask them to help you apply sunscreen to the places you cannot reach.
Otherwise, because your arms just don’t bend that way, you will end up not being able to apply the sunscreen to the inner portions of the left and right quadrants.
You will end up with strange “sunburn wings” as a result.
This is not attractive, and even less comfortable.
This has been your Summer Advice for the day.