public static char charAt(String s, int n,  int m) {        
    if (n == 1) return s.charAt(m);
    char first  = charAt(s, n / 2, m * 2);
    char second = charAt(s, n / 2, m * 2 + 1);
    System.out.print(first + "-" + second + "-");
    return first;
}