text problem

I know how to use text in the game engine. What I want to know is how do you make a text value decrease? So like I start out at 100 and it goes to 90 whenever I get hit by an enemy? I tried using Actuator=Property=Add=-10 but all it does is add -10 to the 100, making it 100-10-10-10… etc. Help?

oops nevermind. I had to change from string to integer.