class M{
  static String c(String s){
    char[] a = s.toCharArray();
    int l = a.length,
            x,
            y;
    return l < 2
            ? s
            : "" + (l%2 > 0
                     ? a[l/2]
                     : (char)((x = a[l/2]) > (y = a[(l/2)-1])
                                ? y + ((x-y)/2)
                                : x + ((y-x)/2)));
  }
 
  public static void main(String[] a){
    System.out.println(c("12345"));
    System.out.println(c("Hello"));
    System.out.println(c("Hiya"));
    System.out.println(c(""));
    System.out.println(c("x")); // Additional test case that will fail on some other answers
  }
}

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