int diff = 0,t;
    t = s.nextInt();
    String str = s.next();
    int len = str.length();
    for(int k = 0; k < t;k++)
    {    
    if(len%2!=0)
        System.out.println("-1");
    else
    {
        String str1 = str.substring(0,(len/2));
        String str2 = str.substring(len/2);        
        for(int i=0;i<str1.length();i++)
        {

                if(!str2.contains(str1.charAt(i)+""))
                    diff++;                       

        }
                   System.out.println(diff);
    }
   }