/******************************************************************************
Welcome to GDB Online.
GDB online is an online compiler and debugger tool for C, C++, Python, Java, PHP, Ruby, Perl,
C#, OCaml, VB, Swift, Pascal, Fortran, Haskell, Objective-C, Assembly, HTML, CSS, JS, SQLite, Prolog.
Code, Compile, Run and Debug online from anywhere in world.
*******************************************************************************/
#include <bits/stdc++.h>
using namespace std;
#define endl "\n"
#define int long long
int fre[500];
vector <int> v[500],arr;
int pre[2000000];
void construct(char a,char b){
int i=0,j=0;
arr.clear();
while(i!=v[a].size()||j!=v[b].size()){
if(i==v[a].size()){
arr.push_back(-1);
j++;
}
else if(j==v[b].size()){
arr.push_back(1);
i++;
}
else if(v[b][j]>v[a][i]){
arr.push_back(1);
i++;
}
else{
arr.push_back(-1);
j++;
}
}
return;
}
int calc(){
int n=arr.size(),st=0,mn=1e9,ans=0;
pre[0]=arr[0];
for(int i=1;i<n;i++) pre[i]=pre[i-1]+arr[i];
for(int i=0;i<n;i++){
if(arr[i]==-1){
for(int j=st;j<i;j++) mn=min(mn,pre[j]);
mn=min(mn,0ll);
st=i;
}
ans=max(ans,pre[i]-mn);
}
return ans;
}
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
int n,ans=0;
cin>>n;
string s;
cin>>s;
for(int i=0;i<s.size();i++){
v[s[i]].push_back(i);
fre[s[i]]++;
}
for(int c1='a';c1<='z';c1++){
for(int c2='a';c2<='z';c2++){
if(c1==c2||fre[c1]==0||fre[c2]==0) continue;
construct(c1,c2);
ans=max(ans,calc());
}
}
cout<<ans<<endl;
return 0;
}
