Add numbers

i have a text box with a number, and then i want to add another number, i named the text var number

on (release) {
 number = number + 3
}

let’s imagine that the text box have the number 2, when i press the button i get 23 and not 5, because it just add a number in front of the one that was already there, and not 2+3=5

some help ??