#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define ff(i,a,b) for(int i=a;i<=b;i++)
#define fb(i,b,a) for(int i=b;i>=a;i--)
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef pair<int,int> pii;
const int maxn = 1000005;
const int inf = 1e9 + 5;
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// os.order_of_key(k) the number of elements in the os less than k
// *os.find_by_order(k) print the k-th smallest number in os(0-based)
int n;
string s;
vector<int> vec[maxn];
int pref[maxn][26];
int main()
{
ios::sync_with_stdio(false);
cout.tie(nullptr);
cin.tie(nullptr);
cin >> n >> s;
s = ' ' + s;
ff(i,1,n){
vec[s[i] - 'a'].pb(i);
pref[i][s[i] - 'a'] += 1;
ff(j,0,25)pref[i][j] += pref[i - 1][j];
}
// ne moze ovako treba ono koje se najvise i najmanje pojavljuje
int rez = 0;
ff(a,0,25){
// za slovo a
if(vec[a].empty())continue;
ff(b,0,25){
if(vec[b].empty() || a == b)continue;
int mn = 0;
ff(i,0,sz(vec[a]) - 1){
int id = vec[a][i];
int kol = i + 1 - pref[id][b] - mn;
rez = max(rez, kol);
mn = min(mn, i + 1 - pref[id][b]);
}
}
}
cout << rez << endl;
return 0;
}
/**
// probati bojenje sahovski ili slicno
**/
