#include<stdio.h>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
 
 
using namespace std;
 
//conversion
//------------------------------------------
inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}
 
//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}
 
//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
 
//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
 
//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
 
//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);
 
//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))
 
//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

bool all_reachable(const std::vector<std::vector<int> >& adjacencies, int start_idx, int end_index) {
    std::vector<bool> visited(adjacencies.size(), false);
    std::queue<int> queue;
    queue.push(start_idx);
    while (!queue.empty()) {
        int node_idx = queue.front();
        queue.pop();
        visited[node_idx] = true;

        for (int neighbour_idx : adjacencies[node_idx]) {
            if (!visited[neighbour_idx]) {
                queue.push(neighbour_idx);
            }
        }
    }
    return visited[end_index];
}

void cut_edge(VVI& adj, int start_idx, int end_index){
  for(auto it = begin(adj[start_idx]); it!=end(adj[start_idx]); ++it){
    if(end_index == *it){
      adj[start_idx].erase(it);
      break;
    }
  }
  for(vector<int>::iterator it = adj[end_index].begin(); it!=adj[end_index].end(); ++it){
    if(*it == start_idx){
      adj[end_index].erase(it);
      return;
    }
  }
}

void dispGraph(VVI& adj){
  for(auto i : adj){
    for(auto j : i){
        cout << j << " ";
    }
    cout << "\n";
  }
}

int main(void){
  int n;
  int table[100];
  int ans = 0;
  string s;
  cin>>n>>s;
  REP(i,n-1){
    int c =0;
    REP(j,100)table[j] = 0;
    string a,b;
    a = s.substr(0,i+1);
    b = s.substr(i+1,n-i-1);
    REP(j,a.length()){
      if(table[a[j] - 'a']==0)table[a[j]-'a']++;
    }
    REP(j,b.length()){
      if(table[b[j]-'a']==1){
        c++;
        table[b[j]-'a']=0;
      }

      
    }
    ans=max(ans,c);
  }
  cout<<ans<<endl;
  return 0;
}


Measured memory_bytes: 256