import core.bitop;
import std.algorithm;
import std.ascii;
import std.bigint;
import std.conv;
import std.functional;
import std.math;
import std.numeric;
import std.range;
import std.stdio;
import std.string;
import std.random;
import std.typecons;
import std.container;

alias sread = () => readln.chomp();
alias Point2 = Tuple!(long, "y", long, "x");

T lread(T = long)()
{
    return readln.chomp.to!T();
}

T[] aryread(T = long)()
{
    return readln.split.to!(T[])();
}

void scan(TList...)(ref TList Args)
{
    auto line = readln.split();
    foreach (i, T; TList)
    {
        T val = line[i].to!(T);
        Args[i] = val;
    }
}

void main()
{
    long n = lread();
    string s = sread();
    long max_same = 0;
    foreach (i, e; s)
    {
        long[] lhs_alf = new long[26];
        long[] rhs_alf = new long[26];
        string lhs_str = s[0 .. i];
        string rhs_str = s[i .. $];
        input_to_array(lhs_str, lhs_alf);
        input_to_array(rhs_str, rhs_alf);
        long same = count_same(lhs_alf, rhs_alf);
        max_same = max(same, max_same);
    }
    writeln(max_same);

}

void input_to_array(string s, long[] a)
{
    long key;
    foreach (i, e; s)
    {
        key = e - 'a';
        a[key] = 1;
    }
}

long count_same(long[] a, long[] b)
{
    long count;
    foreach (e; 0 .. a.length)
    {
        if(a[e] && b[e])
        {
            count++;
        }
    }
    return count;
}