using System;
using System.Collections.Generic;
using static Exter;
using System.Linq;
#if DEBUG
#else
using console = System.Console;
#endif

public class App
{
    public static void Main(string[] args)
    {
        console.ReadLine();
        string s = console.ReadLine();
        int[] cc = new int[26];//charactors count
        foreach(var c in s)
            cc[c-'a']++;
        int[] ccc = new int[26];//cutted charactors count
        int cccc = 0;//current calculated charactors count
        int ccccc = 0;
        foreach(var c in s)
        {
            if(ccc[c-'a'] == 0)
                cccc++;
            if(cc[c-'a'] == 1)
                cccc--;
            cc[c-'a']--;
            ccc[c-'a']++;
            if(ccccc < cccc)
                ccccc = cccc;
        }
        console.WriteLine(ccccc);
    }
}
public static class Exter
{
    public class Point
    {
        public Point(int cx, int cy)
        {
            x = cx;
            y = cy;
        }
        public Point(long[] carr)
        {
            x = carr[0].ToInt();
            y = carr[1].ToInt();
        }
        public void Swap()
        {
            int p = x;
            x = y;
            y = p;
        }
        public int x;
        public int y;
    }
    public static void Rep(int loop,Action<int> act)
    {
        for(int i=0;i<loop;i++)
            act(i);
    }
    public static IEnumerable<Result> ListAll<T1,T2,Result>(IEnumerable<T1> a,IEnumerable<T2> b,Func<T1,T2,Result> f)
    {
        foreach(var v in a)
        {
            foreach(var w in b)
               yield return f(v,w);
        }
    }
    public static IEnumerable<Result> ListAll<T1,T2,Result>(IEnumerable<T1> a,IEnumerable<T2> b,Func<T1,T2,Result> f,Func<T1,T2,bool> s)
    {
        foreach(var v in a)
        {
            foreach(var w in b)
               if(s(v,w)) yield return f(v,w);
        }
    }
    public static int ToInt(this long v)=>(int)v;
    public static int ToInt(this string v)=>int.Parse(v);
    public static int Max(params int[] p)=>p.Max();
    public static long Max(params long[] p)=>p.Max();
    public static int Min(params int[] p)=>p.Min();
    public static long Min(params long[] p)=>p.Min();
    public static int Gcd(int a, int b)=>a > b ? GcdRecursive(a, b) : GcdRecursive(b, a);
    private static int GcdRecursive(int a, int b)=>b == 0 ? a : GcdRecursive(b, a % b);
    public static object cout
    {
        set
        {
            console.Write(value.ToString());
            console.WriteLine();
            
        }
    }
    public static void ToCout<t>(this IEnumerable<t> v)
    {
        cout = string.Join(",",v);
    }
    public static long[] readarr
    {
        get 
        {
            preadarr = console.ReadLine().Split().Select(s=>long.Parse(s)).ToArray();
            return preadarr;
        }
    }
    public static long[] preadarr;
}


#if DEBUG
public static class console
{
    static int cursorLine = 0;
    static readonly string[] cin = new string[]
    {
                "6 6",
"aabbccadeaed"
    };
    static public string ReadLine()
    { 
        return cin[cursorLine++];
    }
    static public void WriteLine()
    {
        Console.WriteLine();
    }
    static public void WriteLine<T>(T input)
    {
        Console.WriteLine(input);
    }
    static public void Write<T>(T input)
    {
        Console.Write(input);
    }
    static public void WriteLine(string s,params object[] arr)
    {
        Console.WriteLine(s,arr);
    }
}
#endif


Measured memory_bytes: 13140