using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;
using System.Numerics;

namespace AtCoderWorkspace
{
    public class Solver
    {
        public void Solve()
        {
            var cin = new Scanner();
            var n = cin.nextInt();
            var s = cin.next();

            var ans = 0;
            for (int i = 1; i < n-1; i++)
            {
                var x = s.Substring(0, i);
                var y = s.Substring(i, s.Length-i);

                var cc = 'a';
                var tmp = 0;
                for (int j = 0; j < 26; j++)
                {
                    if (x.Contains(cc) && y.Contains(cc)) tmp++;
                    cc++;
                }
                ans = Math.Max(ans, tmp);
            }

            Console.WriteLine(ans);
        }
    }

    public static partial class MathEx
    {
        public static int GCD(int n, int m) { return (int)GCD((long)n, m); }

        public static long GCD(long n, long m)
        {
            n = Math.Abs(n);
            m = Math.Abs(m);
            while (n != 0)
            {
                m %= n;
                if (m == 0) return n;
                n %= m;
            }
            return m;
        }

        public static long LCM(long n, long m) { return (n / GCD(n, m)) * m; }

        public static bool[] Sieve(int max, List<int> primes = null)
        {
            var isPrime = new bool[max + 1];
            for (int i = 2; i < isPrime.Length; i++) isPrime[i] = true;
            for (int i = 2; i * i <= max; i++)
                if (!isPrime[i]) continue;
                else for (int j = i * i; j <= max; j += i) isPrime[j] = false;
            if (primes != null) for (int i = 0; i <= max; i++) if (isPrime[i]) primes.Add(i);

            return isPrime;
        }
        public static bool IsPrime(int num)
        {
            if (num < 2) return false;
            else if (num == 2) return true;
            else if (num % 2 == 0) return false;

            double sqrtNum = Math.Sqrt(num);
            for (int i = 3; i <= sqrtNum; i += 2)
            {
                if (num % i == 0)
                {
                    return false;
                }
            }

            return true;
        }

        private static readonly List<int> facs = new List<int> { 1 };
        public static int Fac(int n)
        {
            for (int i = facs.Count; i <= n; ++i)
            {
                facs.Add((int)(Math.BigMul(facs.Last(), i) % 1000000007));
            }
            return facs[n];
        }

        public static int Ncr(int n, int r)
        {
            return (n < r) ? 0
            : (n == r) ? 1
            : Fac(n) / (Fac(r) * Fac(n - r));
        }

    }

    class Scanner
    {
        string[] s;
        int 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 int[] DPArrayInt(int N, int ex)
        {
            int[] Array = new int[N + ex];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextInt();
            }
            return Array;
        }

        public long nextLong()
        {
            return long.Parse(next());
        }

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

        public long[] DPArrayLong(int N, int ex)
        {
            long[] Array = new long[N + ex];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextLong();
            }
            return Array;
        }

        public double nextDouble()
        {
            return double.Parse(next());
        }


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

    class Program
    {
        static void Main(string[] s) => new Solver().Solve();
    }
}



Measured memory_bytes: 13268