Helmet slot damage detection problem

[Mortimer Kerman]

Hello, I'm new to mod programming, and I have a problem: I'd like to set a variable "Mask" to a number that depends on the damage of a piece of armor (the helmet). I almost finished the code, but the checker I put did not work. Could someone help me to correct the problem? Thank you in advance. Sorry for the gross mistakes. Line to be corrected:

			MineterstellarVariables.mask = (double) (itemstack.getItemDamage());

The name of armor is:

mineterstellar.item.ItemSpacesuit

ItemSpacesuit.helmet

PS: My mod is for 1.12.