How can not repeat a random number

[kofa]

hello,

How can not repeat a random number




button code:

on (release) {
var n:int=Math.randomInt(20) ;
kofa = n ;    
MC.loadMovie("pic" add n add ".jpg");
// here i need code 
//to disable repeat numbers
// it means if n = 2 
// the number 2 dont show again
}