#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using ld=double;
using vll=vector<ll>;
#define FOR(i,a,b) for(ll i=a;i<(ll)b;++i)
#define ROF(i,a,b) for(ll i=a;i>=(ll)b;--i)
#define PB push_back

#define L 20

int main(){
  ios::sync_with_stdio(0);cin.tie(0);

  map<array<bool, L>, int64_t> m;
  int64_t N;
  cin >> N;
  vector<int> in(N, 0);
  for(int64_t i = 0; i < N; i++){
    char IN;
    cin >> IN;
    in[i]= IN - 'a';
  }
  array<bool, L> cur_pref;
  for(int64_t i = 0;i < L; i++)cur_pref[i] = 0;
  for(int64_t i = 0;i < N+1; i++){
    m[cur_pref] = i;
    if(i < N){
      cur_pref[in[i]] =! cur_pref[in[i]];
    }
  }
  
  
  for(int64_t i = 0;i < L; i++)cur_pref[i] = 0;

  int64_t mx = 0;
  for(int64_t i = 0;i < N; i++){
    mx = max(mx, m[cur_pref] - i);
    for(int64_t j = 0; j < L; j++){
      cur_pref[j] =! cur_pref[j];
      auto it = m.find(cur_pref);
      if(it != m.end()){
        mx = max(mx, it->second - i);
      }
      cur_pref[j] =! cur_pref[j];
    }
    cur_pref[in[i]] =! cur_pref[in[i]];
  }
  cout << mx << endl;

  return 0;
}