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

// dfmt off
T lread(T = long)(){return readln.chomp.to!T();}
T[] lreads(T = long)(long n){return generate(()=>readln.chomp.to!T()).take(n).array();}
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;}}
alias sread = () => readln.chomp();enum MOD = 10 ^^ 9 + 7;
alias PQueue(T, alias less = "a<b") = BinaryHeap!(Array!T, less);
// dfmt on

void main()
{
    long N = lread();
    auto S = sread();
    auto C = new uint[](N);
    foreach (i; 0 .. N)
    {
        C[i] = (1 << (S[i] - 'a'));
    }
    long ans = -1;
    foreach (i; 1 .. N)
    {
        uint a = (C[0 .. i + 1] ~ 0).reduce!"a|b"();
        uint b = (C[i + 1 .. $] ~ 0).reduce!"a|b"();
        // writefln("%030b %030b %030b", a, b, a & b);
        ans = ans.max(popcnt(a & b));
    }
    writeln(ans);
}
