Get the last n characters java

public String laynkitucuoi(String s){
        String str = "";
        char[] c = s.toCharArray();
        int dem = 1;
        dem = Integer.parseInt(tfnbatki.getText());
        for(int i = s.length()-1; i > 0;i--){//123456  3  654
            if( dem < i){
                str = str.concat(c[i] +"");
            }
        }  
        return str;
    }