Private and Global variables question

Hi there!i`m with a question in my mind, how do i decrease a number from the global variable?

I added a private variable to the player sprite that show his lifes on a text, but when he go to other layout, the number goes automaticaly to 3 (the initial number)! I thinked a global variable could help...

Can you help me with this task?

go into system and select "subtract from global variable"

thanks