JS AlPHABET Shifting Cipher Randomized V1

<script>
var possible = "ABCGDEFGHIJKLMNOP0QRSTUVWXYZ";
var text;
var am=prompt("Enter Am:");
for(var i2=0;i2<am;i2++){
for(var i=0; i<26; i++){
text=possible.charAt(Math.floor(Math.random()*possible.length))
}
document.writeln(text);
}
</script>