using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        string s = Console.ReadLine();

        Program pro = new Program();

    int num =   pro.secondHighestInteger(s);

        Console.WriteLine(num);

    }

    public int secondHighestInteger(string s)

    {

        int num=0;

        char[] ch=s.ToCharArray();

        int[] array=new int[ch.Length];

        int count=0;

        for(int i=0;i<ch.Length;i++)

        {

            if(Char.IsNumber(ch[i])){

                array[count]= (int) Char.GetNumericValue(ch[i]);

                count++;

            }

        }

        if(count==0)

        {

            return -1;

        }

        int[] newrray=array.Distinct().ToArray();

        Array.Sort(newrray);

        num=array[newrray.Length-1];

        return num;

    }

}