import std.stdio, std.string, std.conv;
import std.range, std.algorithm, std.array, std.typecons, std.container;
import std.math, std.numeric, core.bitop;


void main() {
    int n;
    scan(n);
    auto s = readln.chomp.to!string;

    auto pf = new int[](n + 1);
    foreach (i ; 1 .. n + 1) {
        pf[i] = pf[i-1] | (1 << (s[i-1] - 'a'));
    }

    auto sf = new int[](n + 1);
    foreach_reverse (i ; 0 .. n) {
        sf[i] = sf[i+1] | (1 << (s[i] - 'a'));
    }

    int ans;

    foreach (i ; 0 .. n) {
        ans = max(ans, popcnt(pf[i] & sf[i]));
    }

    writeln(ans);
}





































void scan(T...)(ref T args) {
    import std.stdio : readln;
    import std.algorithm : splitter;
    import std.conv : to;
    import std.range.primitives;

    auto line = readln().splitter();
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}



void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}
