Why is my script only increasing its Z axis by 10m?

key alt;
string target_alt;

default
{
    state_entry()
    {
        llListen(1,"",llGetOwner(),"");
    }
    listen(integer channel, string name, key id, string message) {
        if (llGetSubString(message, 0, 3) == "/alt") {
           target_alt = llToLower(llGetSubString(message,5,-1));
           llSay(0,"Altitude " + target_alt + " Meters");
}
        if (message == "build")
        {
            llSetPos(llGetPos() + <0.0,0.0,(float)target_alt> );
            llOwnerSay("Done!");
        }
    }
}