#include <iostream>
#include <cstdint>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <cassert>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <unordered_set>
#pragma GCC optimize("Ofast")

using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int, int> > vpii;
typedef vector<vector<int> > vvi;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<ll> > vvll;

#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define irep(it, stl) for (auto it = stl.begin(); it != stl.end(); it++)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define mes(a) cout << (a) << endl
#define dmes(a, b) cout << (a) << " " << (b) << endl

#define CHOOSE(a) CHOOSE2 a
#define CHOOSE2(a0,a1,a2,a3,a4,x,...) x
#define debug_1(x1) cout<<#x1<<": "<<x1<<endl
#define debug_2(x1,x2) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<endl
#define debug_3(x1,x2,x3) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<endl
#define debug_4(x1,x2,x3,x4) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<", "#x4<<": "<<x4<<endl
#define debug_5(x1,x2,x3,x4,x5) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<", "#x4<<": "<<x4<<", "#x5<<": "<<x5<<endl
#define debug(...) CHOOSE((__VA_ARGS__,debug_5,debug_4,debug_3,debug_2,debug_1,~))(__VA_ARGS__)

#define ynmes(a) (a) ? mes("Yes") : mes("No")
#define YNmes(a) (a) ? mes("YES") : mes("NO")
#define re0 return 0
#define mp(p, q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define rSort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
#define itn int;
int dx[8] = { 1, 0, -1, 0, 1, -1, -1, 1 };
int dy[8] = { 0, 1, 0, -1, 1, 1, -1, -1 };
template <class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template <class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
struct io { io() { ios::sync_with_stdio(false); cin.tie(0); } };
const int INF = INT_MAX;
const ll LLINF = 1LL << 60;
const ll MOD = 1000000007;
const double EPS = 1e-9;

ll gcd(ll a, ll b) {
    return b ? gcd(b, a%b) : a;
}

ll lcm(ll a, ll b) {
    return a / gcd(a, b) * b;
}

ll nlcm(vector<ll> numbers) {
    ll res;
    res = numbers[0];
    for (ll i = 1; i < (ll)numbers.size(); i++) {
        res = lcm(res, numbers[i]);
    }
    return res;
}


signed main() {
    ll n;
    cin >> n;
    string s;
    cin >> s;
    
    map <char, ll> mp1;
    map <char, ll> mp2;
    
    ll r = 0;
    ll tmp = 0;
    
    for(ll i=1; i < n; i++) {
        for(ll j=0; j < i; j++) {
            mp1[s[j]]++;
        }
        
        for(ll j=i; j < n; j++) {
            mp2[s[j]]++;
        }
        for (auto k: mp1) {
            if (k.second > 0 && mp2[k.first] > 0) tmp++;
        }
        
        rep(k, 26) {
            mp1[97+k] = 0;
            mp2[97+k] = 0;
        }
        
        
        r = max(r, tmp);
        tmp = 0;
        
    }
    
    mes(r);
    
    
    
}


Measured memory_bytes: 3588