Expressions Returning NaN

Hello, I have an expression that takes 3 values of type double and should return an angle. Instead, the expression returns NaN.
How can I assure that the expression doesn’t return NaN?
NaN_Error.gh (3.7 KB)

Hello
An ACos(x) can’t return a value if x is less than -1 or more than 1 !
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Maybe you need to use rad

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Tysm! I had to put 2ac in parentheses, but your reply helped nonetheless.