#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#define ll long long
#define u_ll unsigned ll
#define pb push_back
#define F first
#define S second
#define all(x) (x).begin(), (x).end()
using namespace std;
#define MAXN 210000
#define MOD 1000000007
#define INF 100000000000000000ll
typedef tree<ll, null_type, less_equal<ll>,
             rb_tree_tag, tree_order_statistics_node_update>
    TREE;

ll bpow(ll b, ll e);
ll res(ll a, ll b) { return ((a % MOD) - (b % MOD)) % MOD; }
ll sum(ll a, ll b) { return ((a % MOD) + (b % MOD)) % MOD; }
ll mult(ll a, ll b) { return ((a % MOD) * (b % MOD)) % MOD; }
ll invmod(ll a) { return bpow(a, MOD - 2); }
ll divmod(ll a, ll b){ return mult(a,invmod(b)); }

ll bpow(ll b, ll e)
{
    ll rets[3] = {1,b,mult(b,b)};
    if(e < 3)return rets[e];
    ll t = bpow(b,e/2);
    t = mult(t,t);
    if(e&1)
        t = mult(t,b);
    return t;
}

int n,c;
vector<vector<pair<ll,ll>>> g;

ll dfs(int x,int p = -1) {
    bool ok = 0;
    ll mn = 0;
    for(auto to: g[x]){
        if(to.first == p)continue;
        mn += min(to.second,dfs(to.first,x));
        ok = 1;
    }
    if(ok == 0){
        mn=MAXN;
    }
    //cerr << (x+1) << " " << mn << " " << mx << endl;
    return mn;
}

signed main()
{   
    int n;
    cin >> n ;
    string s;
    cin >> s;
    vector<int> dp(28);

    vector<int> dp2(28);
    for(int i = 0 ; i < n; i++){
        dp[s[i]-'a']++;
    }
    ll mx = 0;
    for(int i = n-1; i >= 0; i--){
        dp[s[i]-'a']--;
        dp2[s[i]-'a']++;
        ll cnt = 0;
        for(int i = 0; i < 28; i++){
            if(dp[i] > 0 && dp2[i] > 0){
                cnt++;
            }
        }   
        mx = max(mx,cnt);
    }

    cout << mx << endl;
}