using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

namespace AtCoder.ABC098.B
{
    class Program
    {
        static void Main ()
        {
            var sc = new Scanner ();
            int n = sc.Nextint ();
            var s = sc.Next ();
            int max = 0;
            for (int i = 1; i <= n - 1; i++)
            {
                var s1 = s.Substring (0, i).ToCharArray ().Distinct ();
                var s2 = s.Substring (i, n - i).ToCharArray ().Distinct ();
                var commonch = s1.Intersect<char> (s2).ToArray ();
                max = Math.Max (max, commonch.Length);
            }
            Console.WriteLine (max);
        }
    }

    class Scanner
    {
        string[] s;
        long i;

        char[] cs = new char[] { ' ' };

        public Scanner ()
        {
            s = new string[0];
            i = 0;
        }

        public string Next ()
        {
            if (i < s.Length) return s[i++];
            string st = Console.ReadLine ();
            while (st == "") st = Console.ReadLine ();
            s = st.Split (cs, StringSplitOptions.RemoveEmptyEntries);
            if (s.Length == 0) return Next ();
            i = 0;
            return s[i++];
        }

        public int Nextint ()
        {
            return int.Parse (Next ());
        }
        public int[] Arrayint (int N, int add = 0)
        {
            int[] Array = new int[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = Nextint () + add;
            }
            return Array;
        }

        public long NextLong ()
        {
            return long.Parse (Next ());
        }

        public long[] ArrayLong (int N, long add = 0)
        {
            long[] Array = new long[N];
            for (long i = 0; i < N; i++)
            {
                Array[i] = NextLong () + add;
            }
            return Array;
        }

        public double NextDouble ()
        {
            return double.Parse (Next ());
        }

        public double[] ArrayDouble (long N, double add = 0)
        {
            double[] Array = new double[N];
            for (long i = 0; i < N; i++)
            {
                Array[i] = NextDouble () + add;
            }
            return Array;
        }

        public decimal NextDecimal ()
        {
            return decimal.Parse (Next ());
        }

        public decimal[] ArrayDecimal (long N, decimal add = 0)
        {
            decimal[] Array = new decimal[N];
            for (long i = 0; i < N; i++)
            {
                Array[i] = NextDecimal ();
            }
            return Array;
        }
    }

    static class MathExtensions
    {
        public static long Factorial (long n)
        {
            long fact = n;
            for (long i = n - 1; i >= 1; i--)
            {
                fact *= i;
            }
            return fact;
        }

        static long Combination (long n, long r)
        {
            long molecule = 1;
            for (long i = 0; i < r; i++) molecule *= n - i;
            long denominator = 1;
            for (long i = 1; i <= r; i++) denominator *= i;
            return molecule / denominator;
        }
    }

    static class IEnumerableExtensions
    {
        public static long Nearest (this IEnumerable<long> self, long target)
        {
            var min = self.Min (val => Math.Abs (val - target));
            return self.First (val => Math.Abs (val - target) == min);
        }
    }
}

Measured memory_bytes: 13396