public static int superDigit(String n, int k) {
        StringBuilder superNBuilder = new StringBuilder(n);
        for(int i=0;i<k-1;i++)
        {
            superNBuilder.append(n);
        }
        String superN = superNBuilder.toString();
        BigInteger result = new BigInteger(""+superN.charAt(0));
        while(superN.length()>1){
            result=BigInteger.ZERO;
            for(int i=0;i<superN.length();i++){
                result=result.add(new BigInteger(""+superN.charAt(i)));
            }
            superN=result.toString();
        }
        return result.intValue();
    }